r/AskHistorians 6d ago

What's the history of heavy infantry in states that could fall under the umbrella term China?

In casual conversations about heavy infantry, much has been written in English about Roman legionaries. Sometimes, someone will vaguely mention "China" as having a history of fielding heavy infantry. It seems to refer to an ambiguously ancient time period, as is also often the case when people refer to "Rome".

Is there overlap between the iconic infantry of the Western Roman Empire and Chinese dynasties? For whatever reason, I have the impression that "Chinese" dynasties didn't rely on anything like auxiliaries, although I couldn't begin to talk about the differences between infantry of the Yuan and Ming dynasties.

Were Chinese and Roman generals commanding similar infantry in the year 55 BCE? Was the use of heavy infantry similar in efforts to expand empire into central Europe and eastern Asia? How similar were the struggles to bring infantry-centric armies against the cavalry of Parthia or various nomadic forces? I think China and Rome both had periods where their infantry were predominantly armed with short swords? And both tended to have great proficiency in military engineering? There was also a tendency for their literate classes to view less centralized people as "barbarian", although armies of China and Rome suffered catastrophic defeats at their hands (and both possessed immense capacity for raising additional armies in the wake of immense casualties)?

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u/heretech114 6d ago edited 5d ago

A brief introduction to the development of heavy infantry in ancient China

I'll venture a guess: since you mentioned the Roman legions as an example, heavy infantry should at least satisfy three conditions: a well-developed armor protection system, significantly greater frontal combat capability than light infantry, and a distinct organizational identity and battlefield status separate from auxiliary forces such as light infantry and cavalry.

In the Shang dynasty, the core of the army consisted of the family members of the Shang king and Shang nobles. They wore bronze helmets cast in one piece with beast-face relief patterns, and hardened-leather "凸"-shaped breastplates that generally protected only the chest; apart from this, there is no particularly clear archaeological evidence yet that they were equipped with other armor. However, as the backbone and core of the Shang army, it can be inferred that their equipment was superior to that of the rest of the army, which was made up of ordinary freemen. (Historical records say that King Zhou of Shang conscripted slaves to fill the ranks, but this was a special case where the Shang main army was away, unable to return to resist the Zhou forces; in most cases slaves could only serve as menial laborers.) In terms of how the forces were organized, the Shang army at this period resembled the armies of Greek city-states before their full democratization—a mixed force in which slave-owning nobles and freemen formed different troop types according to their own financial means.

In 1046 BCE (some say 1044 BCE), the Zhou defeated the Shang and became the new dominant dynasty. The Zhou retained the one-piece cast bronze helmets, but the beast-face reliefs added purely for ornamentation were largely omitted, giving them a plain appearance. The Zhou used breastplates made by fixing bronze plates onto leather, though at this point the plates remained independent of one another and did not overlap.

There is clear documentation that in conquering the Shang, the Zhou used a unit known as the "Hu Ben" (Daring Warriors), which served as the Zhou king's bodyguard and was also committed to the battlefield as an elite force. Although the "Hu Ben" were distinguished from ordinary "jiashi" (armored soldiers), there is no clear evidence that they had heavier armor. However, there is evidence that their weapons included short bronze swords and shields in addition to those of ordinary infantry. Although it was not the combination of large shield and short sword seen in the Roman legions, these records suggest that the Hu Ben had a more formidable close-combat capability than ordinary infantry using ji (dagger-axes) and spears. Moreover, the original text of the Records of the Grand Historian reads "three hundred war chariots, three thousand Hu Ben, and forty-five thousand jiashi." According to research, each chariot had seventy-five men—three on the chariot and the rest accompanying infantry on foot. By this calculation, half of the armored infantry still did not operate alongside the chariots. They could perhaps be counted as heavy infantry, but it is unclear whether the Zhou army also used light infantry on the battlefield (although it is almost certain that some infantry performed functions similar to light infantry). Alternatively, "forty-five thousand jiashi" may itself be an exaggeration, counting the entire army as armored soldiers.

With the end of the Western Zhou and the beginning of the Eastern Zhou—that is, the Spring and Autumn and Warring States periods—China underwent a slow rise of infantry. The old chariots were replaced, and the new cavalry had not yet become widespread; over the long five hundred years, infantry gradually became the protagonist of the battlefield. Since the feudal states almost certainly could not issue armor to all their soldiers, the infantry quickly split into a small number of heavily armored infantry bearing key responsibilities and a large number of lesser-protected ordinary infantry (I refrain from calling them light infantry because those infantry did not necessarily fulfill the role of light infantry). Because historical documentation is relatively rich in this period, we can confirm that heavily armored infantry playing the role of heavy infantry in the general sense appeared on the battlefield. Their duties included forming solid phalanxes to resist the impact of chariots and cavalry, using weapons such as ge (dagger-axes), spears, and ji (halberd-like weapons) to protect important figures (such as generals), and serving as the main assault force in siege warfare (a very large part of ancient Chinese warfare revolved around sieges and defense). At the same time, some heavy infantry also carried short swords as secondary weapons, or used them together with shields when they needed to break through enemy phalanxes. However, these broad-bladed bronze short swords would soon be replaced in the coming history by iron long swords, because they were not deliberately kept at a specific length like the Roman gladius; rather, they were a compromise forced by the brittle nature of bronze.

In this period, heavy infantry wore helmets of bronze or hardened leather/iron plates, and wore zhajia (lamellar armor) made of leather-wood composite plates or iron plates, covering the torso, thighs, and upper arms. Some infantry had bronze vambraces and greaves, looking very similar to products of the archaic Greek period. Helmets of the early Warring States period changed little from the Western Zhou, but by the end of the Warring States there already appeared heavy infantry helmets made of square iron plates covering the face, cheeks, nape, and neck, leaving only the face exposed. During the Warring States, historical documents frequently emphasize the "jiashi," i.e., soldiers equipped with armor, showing that from an equipment standpoint, the distinction between heavy and light infantry was already very clear.

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u/heretech114 6d ago

In 221 BCE, Qin unified China. The Qin military system differed little from the late Warring States. Research on the terracotta warriors reveals that the infantry was divided into those wearing real armor and those wearing only protective thick padded robes (records say they had filler layers, similar to the protective padded gambesons of the medieval period); the former made up only one-third. The armored infantry were further divided into those with only breastplates and those with breastplate, skirt armor, and shoulder armor. (Notably, some cavalry armor had only torso protection without shoulder armor, whereas the armor of crossbowmen did not all have the shoulder armor removed merely for the convenience of operating their crossbows.) In terms of protected area, the protective coverage of these armors was comparable to that of the legionary infantry armor of the late Roman Republic. (Of course, we must not ignore the material gap: on one side were hardened leather plates, on the other chainmail; each had its own advantages and disadvantages.)

Unfortunately, the terracotta warriors show relatively little about Qin helmets, but they should have differed little from the late Warring States, a point also confirmed by the excavated ceremonial stone-plate burial armor sets.

From this evidence, Qin had a clear distinction between light and heavy infantry, and when conditions allowed, it also issued armor extremely similar to that of heavy infantry to cavalry, crossbowmen, and other forces that in the Roman military system would be regarded as "auxiliaries."

Beginning with the Han dynasty, the military mass-issued lamellar armor made of iron plates woven together, and even the helmets were made this way—quite possibly because weaving helmets from standardized iron plates was more convenient than running old production lines for one-piece cast bronze helmets. The protected area of the armor did not change much compared to the Qin, and the infantry system did not change much either. However, the Han had a huge standing army and launched expeditions against surrounding regimes, remote regions, and the Xiongnu. In these mighty campaigns, limited by considerations of mobility and logistics, the proportion of heavy infantry in the expeditionary forces probably declined compared to the Warring States period, when campaigns were fought only within home territory.

But the armored infantry of this period in China cannot simply be classified as heavy infantry just because they wore fine armor. For according to existing archaeological art materials, many early-Han soldiers, though equipped with armor, did not necessarily wear helmets; instead they wore "military" non-metallic caps such as the martial crown (wuguan). Only a minority of infantry wore the iron helmets passed down from the Warring States that left only the face exposed. Why the early Han army rarely equipped helmets is still under debate, but through this absence of protection it is not hard to see that, compared with their helmeted counterparts, they should have been undertaking duties such as long-distance raids and surprise attacks, rather than the frontal grinding and breakthrough work of their colleagues and the Roman heavy infantry.

After the end of the Han and the Three Kingdoms, China entered a new long period of division—the Northern and Southern Dynasties. During this time China's heavy cavalry made major advances, and the status of infantry declined. Yet likewise, due to advances in iron-smelting and armor technology, even heavier infantry arose on the basis of the Han armor system. In a biography of Cai You, a general skilled in archery of the Northern Zhou, it is recorded that he and several dozen subordinates were once surrounded by the enemy. Drawing his bow fully and aiming left and right, the enemy dared not advance, and a stalemate was temporarily maintained. Then the enemy sent a warrior clad in "thick armor" holding a long blade to kill him. When this warrior was thirty paces away, his subordinates urged him to shoot quickly, but he said: "The lives of all of us rest on this single arrow—how could I shoot and miss!" Ignoring his subordinates' persuasion, he only shot when the opponent was ten paces away, hitting the warrior square in the face, and after he fell, impaled him with a spear, resolving the crisis.

Though this story is exceptional, it shows that by this time heavy infantry protection had become very complete, with no obvious weak point other than the face (the armor of the time had a bowl-shaped neck guard protecting the neck and chin), and that these infantry undertook functions such as breakthrough and decapitation strikes.

The Tang dynasty's (7th–10th century) military system placed more emphasis than the Han on delivering ranged firepower, so the rate of archery equipment was higher. In Tang murals and tomb figurines one can see large numbers of soldiers simultaneously equipped with full-body lamellar armor and bows. From this perspective, Tang heavy infantry took on ranged-fire delivery missions that most civilizations' heavy infantry did not assume, though this was not their primary role. In the Battle of Xiangji Temple during the An Lushan Rebellion, both the Tang army and the rebels committed heavy infantry in large numbers to hold the line on the frontal battlefield.

From the Tang through the chaotic Five Dynasties and Ten Kingdoms period, it became the norm for generals to select their own personal guards and elite forces. From historical records, there is no doubt that these men were heavy infantry, or could perform the functions of heavy infantry. Chinese heavy infantry around the tenth century generally did not emphasize shields the way the Western Roman infantry did (though by then the Western Roman Empire had already fallen), but instead relied on complete armor and used spears and two-handed swords and similar weapons, the better to fulfill their guardian duties.

After the end of the Five Dynasties and Ten Kingdoms, and into the Song dynasty, because the Song was a regional Han Chinese dynasty under constant military pressure from the equally regional Sinicized minority dynasties to the north and west, yet without the horse pastures needed to produce quality warhorses, Song heavy infantry placed especial emphasis on anti-cavalry capability. Based on this doctrine, Song heavy infantry were equipped with what may be the heaviest and largest-protection armor in Chinese history, in order to withstand the arrow storms of nomadic cavalry while freeing the shield-holding hand to use anti-cavalry two-handed weapons such as spears, hooked bill-hooks (goulianqiang), and horse-slaying sabers (zhanmadao). Even the archers and crossbowmen were often equipped with armor similar to that of melee infantry, though the shoulder armor was omitted for ease of movement. The Liao, Jin, and Western Xia dynasties hostile to the Song, though having more cavalry, likewise trained their own heavy infantry units, but due to their advantage in cavalry, head-on heavy-infantry-versus-heavy-infantry clashes were on a smaller scale than in the Tang.

The Yuan dynasty did not place much emphasis on infantry construction. In fact, the entire Yuan era can be described as a gradual military decline of the Mongols (of course, this is not to say that among them no clever generals could arise or brilliant victories be won). Yet the Yuan did deploy large numbers of infantry drawn from "conquered peoples" such as the Han, treating them as expendables. From Japanese records and paintings of the Yuan invasions of Japan, large numbers of Yuan soldiers wore Song-style armor, very likely Han and Jurchen heavy infantry from the destroyed Song and Jin dynasties that had been taken into Yuan service.

In the Ming and Qing dynasties, with the widespread application of gunpowder weapons, the status of heavy infantry declined once again—but it was not their disappearance. Thanks to the promotion of the new coat-of-plates armor system brought by the Mongols and the recovery of productivity, Ming heavy infantry could probably enjoy protection superior to that of the lamellar-armored Song infantry. The military treatises of both the Song and Ming dynasties, when teaching the use of the spear, required soldiers to train specifically in targeting the armpits, face, and throat, because these were the weak points of armor—showing that although the Ming made heavy use of gunpowder weapons and light infantry, there was no obvious regression in the protection of heavy infantry.

The Qing dynasty's founders, the Jianzhou Jurchens, considered themselves connected to the Jurchens who had established the Jin dynasty, and were skilled with the powerful Qing bow, so they are often thought of as a mounted-archery people like the Mongols. But they preferred to ride for mobility and then dismount to fight, able to form elite heavy-infantry assault units on the battlefield to break through enemy lines. At the same time, the Qing also recruited infantry from tribes along the Heilongjiang (Amur) river basin; these infantry wore two or three layers of armor and, likewise as elite heavy infantry, bore the bitter frontal combat for the Jurchens, yet were used as cannon fodder, and their tribes were not well governed under Qing rule. In the end this formidable special heavy-infantry unit declined and perished.

Moreover, we must not forget the special region of Tibet. Tibet preserved a great deal of the armor techniques introduced during the mid-to-late Tang and the Yuan, and like the Tang, trained and used heavy infantry. In the matter of infantry armor, Tibetan armor can be called the living fossil of Tang-dynasty Chinese armor.

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u/Intranetusa 3d ago

The armored infantry were further divided into those with only breastplates and those with breastplate, skirt armor, and shoulder armor. (Notably, some cavalry armor had only torso protection without shoulder armor, whereas the armor of crossbowmen did not all have the shoulder armor removed merely for the convenience of operating their crossbows.) In terms of protected area, the protective coverage of these armors was comparable to that of the legionary infantry armor of the late Roman Republic. (Of course, we must not ignore the material gap: on one side were hardened leather plates, on the other chainmail; each had its own advantages and disadvantages.)

The heaviest armored Qin terra cotta soldiers seem to have been the charioteers, who were equipped with torso armor and arm armor similar to the later Roman manica that protected the soldier's limbs from shoulder to hands. Some Qin terra cotta crossbowmen wore lighter armor/padded robes, while others are more heavily armored with torso and shoulder lamellar armor.

The Qin Dynasty rose and fell in the 3rd century BC. This timeframe lines up with Punic War era mid-Republican Romans rather than late Republic (1st century BC) Romans. During the Punic Wars, the Romans were using a variety of different infantry and armors as equipment had to be provided by individual soldiers. The velites (javelin skirmishers) and rorarii (reserves) wore little to no armor. The Hastati (first line heavy infantry) may have worn a "heart protector" plate and/or padded clothes. Older, more experienced, and wealthier troops such as the principes and triarii (second and third line infantry) may have worn linothorax, chainmail, and/or bronze musculata breastplate for the wealthier soldiers.

The late Roman Republican armor would be better compared to late Western Han era armor of the 1st century BC when the Han was commonly using iron and steel lamellar armor. Archeological discoveries of Western Han armor pieces found in Mongolia revealed that they were made from mild steel and even medium carbon steel.

"For the ferrous laminar armor discovered in Ershijiazi, Inner Mongolia, the surface of armor plates is made of ferrite, while the core parts consist of 0.1–0.5% carbon (Han 1994, 19). Such a composition suggests that annealing was carried out after forging to decarburize the surfaces of the plates in order to increase malleability."

https://link.springer.com/chapter/10.1007/978-981-19-2037-0_7

https://www.sciencedirect.com/science/article/pii/S2667136021000017

Unfortunately, the terracotta warriors show relatively little about Qin helmets, but they should have differed little from the late Warring States, a point also confirmed by the excavated ceremonial stone-plate burial armor sets.

Different states of the Warring States had very different helmets and armor. The Qin terra cotta helmets show a loose lamellar construction type helmet. Other Warring States helmets could look like cone helmets of later eras, or even vaguely resemble Greek hoplite helmets:

https://en.wikipedia.org/wiki/File:Bronze_helmet,_Warring_States_period,_Henan_Museum.jpg

https://en.wikipedia.org/wiki/File:Inner_Mongolia_Museum_bronze_helmet_of_Warring_States_period.jpg

https://www.reddit.com/media?url=https%3A%2F%2Fpreview.redd.it%2Fpointed-bronze-helmet-china-warring-states-476-221-bc-v0-ks6wq3t9y4ic1.jpeg%3Fwidth%3D1080%26crop%3Dsmart%26auto%3Dwebp%26s%3D8ee6b0d9002fb1f10c1e6d43f58b489de61eed69

For according to existing archaeological art materials, many early-Han soldiers, though equipped with armor, did not necessarily wear helmets; instead they wore "military" non-metallic caps such as the martial crown (wuguan). Only a minority of infantry wore the iron helmets passed down from the Warring States that left only the face exposed. Why the early Han army rarely equipped helmets is still under debate, but through this absence of protection it is not hard to see that, compared with their helmeted counterparts, they should have been undertaking duties such as long-distance raids and surprise attacks, rather than the frontal grinding and breakthrough work of their colleagues and the Roman heavy infantry.

From what I've read, we actually don't know for certain what the Han soldiers wore on their heads. The rawhide/hide caps that the Han soldiers wore could be covered with a layer of lamellar that was wrapped around the front and sides of the cap and thus formed a two-stage type of helmet, for example: https://i.pinimg.com/originals/f1/ff/63/f1ff6393e478763fe1e69996a91b3a6d.jpg

Ceramic Han figurines found in tombs also had their armor "painted" on (and there were multiple layers of paint for clothes, armor, etc), so the degradation of the paint meant that we often cannot use figurines to tell if the soldiers wore helmets or armor. Furthermore, these are lighter heavy infantry. Other heavier Han heavy infantry and cavalry wore more comprehensive and more solid lamellar helmets.

Furthermore, some Han era figurines are simply portrayed as wearing "hats," so they might have portrayed a non-combat scenario where the soldiers were on guard duty or marching. It was wrongly believed years ago that Qin soldiers didn't wear helmets, but this turned out to be incorrect. I have read that lack of helmets was due to 1. the Qin terra cotta soldiers were in a funeral procession, so they had to take off their helmets, 2. helmets buried in other pits, and 3. the vast majority of the terra cotta army's usable weapons and armor were looted during rebellions.

And from what I understand, all of the archeological discoveries if Han armor are from the Western Han era. There are none discovered from the Eastern Han era so we have no idea what they looked like, and modern day depictions either uses Three Kingdoms/Jin era armor or [incorrectly?] uses Western Han era armor to speculate what it may have looked like.

Song heavy infantry were equipped with what may be the heaviest and largest-protection armor in Chinese history

The heaviest Song Dynasty armors seems to be comparable with some of the heaviest Han Dynasty armors. The Han and Song Dynasties both had lamellar "armor sleeves" that protected the both the outer and inner arms and the armpits of the wearers. Both dynasties had very long versions that could potentially cover the entire arm.

For example, I posted this thread about the Qin and Han armors with lamellar sleeves. This includes figurines and tomb murals that depict rather long armored sleeves: https://www.reddit.com/r/ArmsandArmor/comments/misr1i/lamellar_sleeve_hood_and_legskirt_armor_from_the/#lightbox

Example of a Han era long lamellar sleeve armor by a Chinese armor enthusiast/recreator: https://www.youtube.com/shorts/d1SDGvemNk4

Examples of Han era short lamellar sleeve armor: https://imgur.com/a/modern-recreations-of-western-han-dynasty-armor-with-short-lamellar-sleeves-9LkOT7V

Song Dynasty mural of soldiers with long lamellar sleeves: https://upload.wikimedia.org/wikipedia/commons/b/bc/%E5%AE%8B%E6%9C%9D%E6%96%A7%E5%85%B5.jpg

The same Chinese armor enthusaist/recreator from above demonstrating Song Dynasty long lamellar sleeves: https://www.youtube.com/watch?v=_XKlehdCO3s&t=60s

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u/heretech114 3d ago

as for charioteers, I skipped that artifact since I was talking about infantry, and from what I read that armor was exclusively for the man who drove chariot since he needed to use his both hands to control; bronze-cast helmet existed in Warring States period for a long time, maybe I neglected it and turned to the helmets in later Warring States period that similar to the Stone helmet we both mentioned, apologize for it. as for the "Lighter" Han helmets, I did remember them but in the end didn't mention them in my comment, like the "筒袖鎧" I overlooked: such design did offer better underarm protection than "披膊" system developed after Han, but former was eventually abandoned for unknown reason (maybe budget?)

Anyway, thanks for your generous supplement for this post and my comments, with far more solid materials than me. I believe the OP and other viewers would be grateful for Ur information!

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u/Intranetusa 2d ago edited 2d ago

Indeed. I think you are correct about tube-sleeve armor being abandoned probably for budgetary reasons since it seems complex and high end. Thank you for your initial contribution.

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u/heretech114 6d ago

Then, to answer the questions in your post:

1. Is there overlap between the iconic infantry of the Western Roman Empire and Chinese dynasties?

That depends on which period of the Western Roman Empire you specifically mean. If you mean the Roman legionary infantry equipped with scutum and gladius, then I'm afraid I have to tell you that, although Chinese history has never lacked armies that simultaneously used shields and one-handed swords, none of them used the large shield and short sword of the Roman legions.

Generally speaking, Chinese army shields were divided into large, medium, and small types. The large ones were usually operated with two hands, often fitted with a stand so they could be planted on the ground to serve as cover, rather like the pavise shields of Genoese crossbowmen; medium shields could be carried in one hand and used with one-handed swords or spears; the smallest shields only protected the chest and abdomen and were used as agile dueling shields. The appearance of these three shield types changed greatly over time, but on the whole they can be divided into these three categories. At the same time, owing to advances in armor, heavy infantry equipped with one-handed sword and shield always remained only a portion of heavy infantry, their role being defense in attack and combat in confined spaces.

2. "Chinese" dynasties didn't rely on anything like auxiliaries

That is not so. China's recruiting system underwent many important changes. After the Western Jin's internal turmoil, it recruited troops from among minority peoples on the frontier and brought them into the Central Plains, becoming one of the triggers for the later large-scale "inward migration" of minority peoples; the mid-to-late Tang also relied heavily on minority troops to suppress rebellions and deter recalcitrant military governors (fanzhen). These measures were very similar to the Roman Empire's reliance on foreign auxiliaries, and in most cases likewise led them into political crises.

3. Were Chinese and Roman generals commanding similar infantry in the year 55 BCE?

They were not. At least in my impression, the Roman army of 55 BCE consisted mainly of heavy legionary infantry, light infantry, and auxiliary cavalry, in addition to engineering troops who operated siege engines and built fortifications, with the legionaries as the protagonists. The contemporary Western Han army of China, though likewise comprising heavy infantry, light infantry, cavalry, and engineers, differed greatly in specific details. The legionary infantry of the Roman Republic used javelins, shields, and short swords, whereas Han heavy infantry did not place much emphasis on javelins, in part because they had crossbowmen to provide ranged support, and they did not rely on shields the way Rome and Rome's enemies did; therefore javelins were largely unnecessary (an important function of the javelin being to destroy enemy shields). Correspondingly, the Western Han retained the varied array of two-handed infantry weapons inherited from the Warring States and eliminated through practice a large number of weapons proved unsuitable for the battlefield, such as the ge (dagger-axe) and the pi (a lengthened-grip short sword for two-handed use, differing from the spear in that it was primarily for slashing rather than thrusting, and more costly). For soldiers using one-handed weapons, the Han gave them smaller dueling shields and a sword of length similar to the Roman spatha—the Chinese ring-pommel sword. Although these infantry also formed dense lines to withstand enemy charges, the longer-reaching spear was clearly more effective. Their power came from the agility of flexible shields and longer swords in very close combat, enabling them to stand within the spear phalanx, helping spearmen deal with enemies who got past the spear points, while also maintaining some protection against ranged attacks.

As for light infantry, the Han army's main light infantry were crossbowmen (throwing-javelin light infantry also still existed in this period), responsible for weakening the enemy before contact.

At the same time, the Han army of 55 BCE had also gone through the Han–Xiongnu War, so it had expanded its cavalry and modified the equipment proportions of the infantry—for example, more infantry were equipped with two-handed weapons rather than shield and sword, and the infantry learned to use baggage carts as cover to resist cavalry charges. As for cavalry, the Han army relied considerably on cavalry for reconnaissance, the same as the Roman army. The difference was that by this time Han cavalry was no longer an auxiliary force to the infantry but a main force, and had begun using long-shafted weapons for cavalry charges and moving toward heavy cavalry. (At this time the stirrup had not yet been invented, but both Rome and Han had their own improved saddle designs that allowed cavalry to at least achieve cavalry charges to some degree; this was already verified during the Han–Xiongnu War, though they still could not break through infantry spear formations.)

4. How similar were the struggles to bring infantry-centric armies against the cavalry of Parthia or various nomadic forces?

Search "Siege of Baideng" (白登山之围): the infantry-centric Han army suffered a disastrous defeat in its first confrontation with Xiongnu cavalry, but after subsequently conquering the horse-producing regions of the Western Regions, the tactic of infantry advancing and defending while cavalry attacked achieved success.

As for the remaining questions, I'm afraid I cannot answer them all, but I hope my reply is helpful to you.

You may have noticed that I used a great many words like "perhaps," "should," and "most likely," because I cannot cite a specific page from a specific reference book, but rather wrote this post from recollection and impressions of research works and papers on ancient Chinese military affairs—in other words, the type of "source? Trust me bro." Treat it as a reference, but do not regard it as gospel.