From the Romulan War
At the start of the twenty-second century unconditional war swept across the Earth. A war that engulfed the great and the small, the rich and the poor, giving no quarter. Each side strove for unconditional victory, and as battle built upon battle the living began to envy the dead.
Chastised by the cataclysm that they had unleashed, the governments of Earth banded together. Humanity vowed to put an end to war, and to strive for the betterment of every living creature.
Start of the 22nd century? 2100? Is this a typo? According to FC, WWIII ended in the 2050s and Earth was united within half a century after that.