Unification Wars
The Unification Wars were a series of conflicts focused on Terra and the rest of the Sol System at the end of the Age of Strife, where the Emperor officially revealed himself to the general populace and began knocking heads to unite Terra and its home system. This is arguably one of if not the most pivotal series of events in the whole Warhammer 40,000 setting, as it marks the Emperor's rise to power, the creation of the Primarchs, the creation of the first Space Marines, and the founding of a united Imperium of Man.
Age of Strife
By M23, the human federation that had spread across the galaxy was on the brink of destruction after the war with the Men of Iron. It's infrastructure and economy ravaged by the war and the loss of it's entire specialized workforce. Already on it's last legs, Warp storms caused by the birth of Slaanesh separated systems from the larger federation. Cut off from the outside, falling into technological regression and vulnerable to xenos aggression, everything fell apart. This was the nail in the coffin for Terra, which was massively overpopulated and no longer had the resources of it's empire to rely on, as well as the depletion of it's own resources including fucking water.
What followed was about 5,000 years of anarchy, as Terra's once united world government quickly fragmented into dozens upon dozens of warring states. Think Mad Max and the Warring States period of China on an apocalyptic scale, where warlords and their warriors known as techno-barbarians warred against one another and subjugated and brutalized those under their domains. To add salt to the wound, chemical and nuclear wars turned the already overtaxed Terra into a barren, polluted wasteland.