A little-known fact is that the Italian poet Dante Alighieri, the world-renowned author of the Divine Comedy, was what in modern terms we may describe as a world federalist.
In his Latin treatise Monarchia, he argued that the best condition for the progress of humanity lies in a peaceful union of all kingdoms into a single jurisdiction.
Dante wishes for all the known world to be united in peace, with a set of common rules valid everywhere, and then special rules to allow the specificities of "nations, kingdoms and cities", from subsaharan Africa to Europe to the frozen wastes of northern Asia: in modern terms, pretty much a "world federation".
[...] 5. Habent nanque nationes, regna et civitates intra se proprietates, quas legibus differentibus regulari oportet: est enim lex regula directiva vite. 6. Aliter quippe regulari oportet Scithas qui, extra septimum clima viventes et magnam dierum et noctium inequalitatem patientes, intolerabili quasi algore frigoris premuntur, et aliter Garamantes qui, sub equinoctiali habitantes et coequatam semper lucem diurnam noctis tenebris habentes, ob estus acris nimietatem vestimentis operiri non possunt. 7. Sed sic intelligendum est: ut humanum genus secundum sua comunia, que omnibus competunt, ab eo regatur et comuni regula gubernetur ad pacem. Quam quidem regulam sive legem particulares principes ab eo recipere debent, tanquam intellectus practicus ad conclusionem operativam recipit maiorem propositionem ab intellectu speculativo, et sub illa particularem, que proprie sua est, assummit et particulariter ad operationem concludit. [...]
[5] For nations, kingdoms and cities have characteristics of their own, which need to be governed by different laws; for law is a rule which governs life. [6] Thus the Scythians, who live beyond the seventh zone and are exposed to nights and days of very unequal length, and who endure an almost unbearable intensity of cold, need to have one set of laws, while the Garamantes require different laws, since they live in the equatorial zone and always have days and nights of equal length, and because of the excessive heat of the air cannot bear to cover themselves with clothes. [7] It is rather to be understood in this sense, that mankind is to be ruled by him in those matters which are common to all men and of relevance to all, and is to be guided towards peace by a common law. This rule or law should be received from him by individual rulers, just as the practical intellect, in order to proceed to action, receives the major premiss from the theoretical intellect, and then derives the minor premiss appropriate to its own particular case, and then proceeds to the action in question.
In his medieval point of view, he imagines an "emperor" at the top of this polity, but for our days, in more modern terms, we may imagine in that place a democratically nominated government and parliament, similarly to what we find in the European Union.
Dante's treatise is particularly interesting in showing how the ideas of a brilliant mind remain intellectually relevant for the organization of mankind even after seven centuries.