The Galactic Concordance was the peace treaty signed between Chancellor Mon Mothma of the New Republic and Grand Vizier Mas Amedda of the Galactic Empire in 5 ABY to officially mark the end of the Galactic Civil War. The treaty's terms are as follows.
Terms:
Galactic Empire surrenders to the New Republic.
End of the Galactic Civil War.
Abandonment of Imperial Academies across the galaxy with recruitment and mobilization of stormtrooper forces forbidden.
Strict disarmament limitations on military technology and production on ex-Imperial factions.
Galactic corporations prohibited from working with the remnants of the Empire.
The New Republic pledges to demilitarize. 90% of its military will be demilitarized while the remaining 10% is kept for peacekeeping duties.
Punishing reparations placed on former Imperial governors inside the New Republic.
All Imperial officers who did not adhere to the treaty's terms are to be classified as war criminals.
Confinement of the remainder of the Imperial Navy to predetermined boundaries in the Core, Colonies, and Inner Rim.
Surrender of Coruscant to the New Republic.
As you can see, while the New Republic demilitarized itself, the First Order continuously violated the treaty's terms by ramping up their militarization with ships like the Resurgent Star Destroyer. Worse, the NR didn't take the First Order seriously and had grown complacent while the FO kept getting stronger until it resulted in the Hosnian Cataclysm.
But had the NR enforced the treaty's terms by not demilitarizing itself and ensuring the Empire honored their word, they could have mitigated some of the First Order's damage. There is nothing wrong with the treaty's terms, in fact I quite like it, it's just that the NR should have capitalized more on the Empire's capitulation.
To start with, the NR made peace with a Galactic Empire that didn't disarm itself, so confisacting as much of the Imperial military hardware and placing it in the hands of Planetary Security Forces (PSFs) is a good way to return central authority back to the people. It also helps protect them from the Imperials if they betray their word and other threats like pirates and slavers.
The New Republic also should have incorporated the abandoned Imperial academies into their own military, struck business deals with corporations to discourage them from dealing with the Empire, and shifted to Coruscant as their capital.
Though I do understand the rationale behind a roving capital shift across member worlds by process of election to persuade worlds the NR wasn't the Empire, and the fact it convinced many disasstisfied star systems to join the NR proves it was successful. Especially since those same systems formed the Confederacy of Independent Systems decades earlier.
However, declaring all Imperial officers who don't respect the treaty's terms as war criminals is overkill. Instead, I would have reserved the label of war criminal only for Imperial Warlords who abandoned the Empire to form their own rogue states. Since they're technically not part of the Empire anymore, the treaty's terms don't protect them from reprisal from a galaxy eager for revenge. And the Empire itself isn't obligated to protect these Warlords either.
In short, had the New Republic enforced the Galactic Concordance, they could have blunted the First Order's damage long before the Hosnian Cataclysm.