Fall of Wall Maria

The Wōru Maria kanraku (ウォール・マリア陥落) was a tragic disaster which began when Shiganshina District came under the attack of the Colossus and Armored Titans, in which humanity was forced to retreat to Wall Rose as the Titans entered humanity's territory for the first time in a hundred years.

Prelude
In the year 830, the nation of Marley began to consider the necessity of claiming the immense amount of fossil fuels on the island of Paradis where the last remnants of the nation of Eldia lived within the three walls Maria, Rose, and Sheena. Over eighty years ago, King Fritz had issued an ultimatum to the nation of Marley at the end of the Great Titan War: if Marley made any move of hostility against the Eldians of Paradis, the King would use the Progenitor Titan to release the countless Colossus Titans contained within the walls. In order to claim Paradis without a direct act of war, the Marleyans chose to begin a warrior program. In this program, low-class Eldian citizens within Marley would offer their children between the ages of 5 and 7 to the Marleyan military, where they would train until seven would be chosen to inherit the seven Titans in Marleyan possession, obtained during the Great Titan War.

Fifteen years later, four Eldian warriors were chosen to be sent out of their "Hometown" to begin the attack on Paradis: Bertolt Hoover, Reiner Braun, Annie Leonhart, and Marcel.