Great Crusade

The Great Crusade was a massive operation launched by the Mother of Mothers following the Imperium's rediscovery of warp technology. The crusade lasted for four hundred years and it saw the start and end of many smaller conflicts, like the Ullanor Crusade, the First indomintus Crusade, and the Concordia Crusade.

Background
The galaxy was plunged into darkness when the warp relays suddenly—and mysteriously—malfunctioned, preventing the ships of the Old Republic from traversing the great void. Trade ceased to exist, and countless planets fell into disarray as their populations rebelled against their masters. Humanity was among the many races affected by the Great Collapse. Until the 7th millennium, humanity had expanded rapidly across the galaxy, colonising world after world. Their homeworld, Terra, was revered as a centre of hub and culture; its skyscrapers grazing the very skin of space itself. Inevitably, when the Solar system was cut off from the rest of the galaxy, the planet fell into ruin. Those who did survive the cataclysmic riots succumbed to hunger as the planet had come to rely on interstellar trade to sustain its massive population. In just a century, the surface of Terra was an irradiated wasteland, its inhabitants mutated beyond comprehension.

However, beneath the rubble lay humanity's greatest hope. Deep within the mountain ranges of the Himalayas, Genetrix—the Mother of Mothers—emerged with an army of custodians. After millennia of conquest, Terra was finally under her rule, and thus she proclaimed the creation of the Imperium of Andarra. Following the conquest of Terra, Genetrix set forth her vast legions of thunder warriors to reclaim the solar system.