The Unification Wars was a series of conflicts focused on Terra and the rest of the Sol System at the tail 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
If you a human at any point it the history of 40K, to be one on Terra during the Age of Strife was probably the shittiest time to live in. By M23, the human federation that had spread across the galaxy was on its last legs after the war with the Men of Iron, infrastructure, economies, and whole populations annihilated by the total destruction of the war and the loss of their entire specialized workforce. Already on it's last legs, Warp storms caused by the Eldar's hedonism and the unborn Slaanesh separated systems from the larger federation, cut off from the outside and falling into technological regression. This was the nail in the coffin for Terra, who was bloated with overpopulation and had little if any of the natural resources of it once had, including fucking water. Nearly completely dependent on outer planets for trade and support, the fall was inevitable.
What followed next was about 5,000 years of anarchy and just a total clusterfuck, 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.