Great Titan War

The Kyojin Taisen (巨人大戦) was a conflict between the people of Marley and the nation of Eldia, ending with the fall of the Eldian Empire and the creation of the Walls.

Prelude
For many hundreds of years, the eight Titan houses subservient to the Fritz family were in constant conflict between each other, though the power of the Progenitor Titan maintained order in the Empire throughout the centuries. However, when the Progenitor Titan was inherited by the 145th King of the Fritz monarchy, he chose instead to abandon the conflict of Eldia and relocate the capital to the island of Paradis to the east of the mainland. In the King's absence, the people of Marley, descendants of an ancient nation defeated by Eldia, seized the opportunity and began civil war.

The War
With their numbers, the people of Marley were able to instigate a civil war, the Great Titan War, in Eldia. At this time, Marley succeeded in capturing seven out of the nine Titans which held dominance in Eldia, dramatically turning the tide of the war in their favor. It is unknown for how long the Great Titan War was waged, but during the conflict Marley succeeded in claiming nearly all the territory that had previously belonged to the Eldian Empire. In the year 743, when Eldia was left with only the island of Paradis as their last undisputed territory, King Fritz chose to make use of the Progenitor Titan to put an end to the war and bring about lasting peace for Eldia. After gathering the people of Eldia, as well as some minority groups, to the island of Paradis, King Fritz used the Progenitor Titan to guide countless Colossus Titans in the construction of three concentric Walls around Eldia's remaining territory: Maria, Rose, and Sheena. When all of Eldia had gathered within the Walls, King Fritz used the power of the Progenitor Titan to erase their memories of the world before the raising of the Walls. For the next century, the people of Eldia would live on believing themselves to be the last remnants of humanity on earth, with the world beyond the Walls overrun by the Titans, who they now believed had appeared without warning in the year the Walls were raised. Once peace was achieved in Eldia, King Fritz sent an official ultimatum to the people of Marley: if they dared to make any hostile move against the people of the Walls, the Colossus Titans within would be unleashed upon the world to "flatten the entire earth". With this, the Great Titan War came to a close.

Aftermath
Across the sea on the mainland of the newly founded nation of Marley, the people of Eldia who were left behind on the continent became low-class citizens under the authority of their new rulers. Rather than exterminating the Eldians, the government of Marley chose instead to confine them within designated ghettos, separate from the Marleyan populace. Some remnants of the Fritz family also remained in Marley, displeased with King Fritz's decision to abandon the conflict of Eldia. Over the next eighty years, some Eldians would come together to form a restorationist movement with the goal of overthrowing Marley and restoring the empire of Eldia.

In the years after the Great Titan War, the Marley government would slander the old Eldian Empire with the claim that they had resorted to eugenics and ethnic cleansing in the 1,700 years before the Great Titan War, when such a catastrophic event had in reality never happened.

One hundred and seven years after the end of the Great Titan War, the Survey Corps discovers the basement of Grisha Yeager, an Eldian of Marley who came to live within the Walls, and in Grisha's journals they learn the true history of the age before the raising of the Walls, as well as Grisha's past before his arrival at Paradis.