|But the once great nation of Marley plotted to subvert Eldia and its arrogance from within. They brought seven of the Nine Titans under their control and were victorious in the Great Titan War 80 years ago.|
|— Grisha's father tells Grisha about the history of Eldia|
The Great Titan War (巨人大戦 Kyojin Taisen?) was a period of civil war within the Eldian Empire. The First King abandoned Eldia's conflicts, which led to fighting between the other eight houses with the power of the Titans. This allowed the oppressed Marleyan people to rise up, and overthrow the weakened Eldia. The war resulted in the fall of the Eldian Empire, the retreat of the Eldians to Paradis Island, the creation of the Walls, and the restoration of Marley.
For many hundreds of years, the various familial houses subservient to the Fritz family were in constant conflict between each other for control over the eight Titans, though the power of the Founding Titan maintained order in the Eldian Empire throughout the centuries. However, when the Founding Titan was inherited by the First Reiss King, he chose instead to abandon the conflict of Eldia and relocate the capital to Paradis Island 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 to take control of the war.
With their numbers, the people of Marley were able to instigate a civil war, the Great Titan War, in Eldia. Amidst the conflict, the nation of Marley succeeded in gaining the powers of 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, the First King chose to make use of the Founding Titan to put an end to the war and bring about lasting peace for Eldia. Eventually, after gathering the people of Eldia, including non–Subjects of Ymir, to Paradis Island, King Fritz used the Founding Titan to guide countless Colossal Titans in the construction of three concentric Walls around Eldia's remaining territory: Maria, Rose, and Sina. In order to cease Marley's advance, King Fritz sent an official ultimatum stating that if they dared to make any hostile move against the people of the Walls, the Colossal Titans within would be unleashed upon the world to "flatten the entire earth." When all of Eldia had gathered within the Walls, the gates were sealed and King Fritz sent a message to the Fritz family who remained in Marley, stating that he had made a vow with the Founding Titan renouncing all war. With this vow, the Great Titan War came to a close.
After the closing of the gates in the Walls, the First King used the power of the Founding Titan to erase the memories of the world before the raising of the Walls from the minds of the Subjects of Ymir. For the next century, the Subjects of Ymir would live on believing themselves to be the last remnants of humanity on earth, with the world beyond the Walls overrun by the Titans that had appeared without warning in the year the Walls were raised. Across the sea on the mainland of the newly reformed 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 the internment zones, such as in Liberio, 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 resistance 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 during their reign, when such a catastrophe might not have ever happened. One hundred and seven years after the end of the Great Titan War, the Scout Regiment discover the basement of Grisha Jaeger, an Eldian of the Marley internment zones who came to live within the Walls; 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 Island.