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
Around twenty years ago, the nation of Marley began to consider the necessity of claiming the immense amount of fossil fuels on the island of Paradis where the majority of the 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.

Shiganshina District
After nearly a century of undisturbed peace, the humans of Shiganshina had become content in their confinement and thought little of the Titans who had been vainly clawing at their doorstop for decades. As such, they thought little of the sacrifices of the Survey Corps who had continued to make ultimately futile efforts to eliminate the Titan presence beyond the Wall, coming back to once again concede that the deaths of their comrades had all been for nothing. The soldiers of the Garrison had become undisciplined and disdainful of their posts, and thus were completely unprepared for the holocaust that was to come.

Finally, in 845, the peace was abruptly ended when the Colossus Titan suddenly appeared at the gates of Shiganshina, much to the horror of the thousands of citizens and soldiers trapped within. The Colossal Titan then proceeded to smash its foot through the outer wall, sending a shower of debris raining across the city and killing dozens of panicked civilians as dozens of mindless Titans began pouring in from the outside. The city descended into chaos as the Titans began to devour the humans by the hundreds, and the Garrison scrambled to evacuate the survivors to the inner wall as they found themselves hopelessly outmatched by the Titans.

During the massacre, Eren Yeager and Mikasa Ackerman rushed back to their home to find that their mother Carla was trapped in the debris and unable to move, and the two children desperately tried to free her without success. Carla pleaded them to save themselves when the soldier Hannes rushed to their aid. Confident he could slay the incoming Titan, Hannes rushed into action hoping to save the entire family, but soon found himself facing the Smiling Titan. Unnerved and losing the will to fight, Hannes raced back to save Eren and Mikasa, against the former's protests even as Carla pleaded them to let her be and to get to safety. Hannes dragged the children away from the house as the Smiling Titan plucked Carla from the wreckage, violently broke her back, and then proceeded to devour her lifeless body, with her son only able to look back in horror.

Unable to contain the invading the Titans, the Garrison evacuated any survivors they could to the gate and packed them into ferries that would take them deeper inside humanity's territory. The situation soon fell completely out of control as thousands fought to escape the city. A small group of soldiers under the command of Hugo assigned cannons to defend the gate, trying to hold off the advancing Titans who largely ignored their fire. As the last ferry was loaded, a completely new breed of Titan covered in armor-like skin appeared, and began charging at the inner wall at top speed. Unable to take it down with cannon fire, the Garrison soldiers panicked and fled for the closing gate, but the Armored Titan then smashed its way through, killing many of the Garrison soldiers and civilians still gathered at the entrance. The final ferry pulled away packed with survivors, leaving hundreds more behind who had been unable to board. The Titans slowly exterminated the doomed population of Shiganshina and then continued into human territory, signaling the fall of Wall Maria.

Aftermath
The loss of Wall Maria was a major disaster for the humans of Paradis, with many thousands lost in the immediate aftermath of the initial attack as the survivors fled towards Wall Rose. The decision was made by the nobility to abandon the entire perimeter of Wall Maria and all adjoining districts, leaving those unable to reach safety to fend for themselves. With nearly a third of their territory lost, humanity was left in a desperate state of affairs, their resources and amount of arable lands greatly reduced as a major refugee crisis took hold.

During the bloodbath, Grisha Yeager raced back towards his home as he had been away when the battle began, desperately trying to rescue his family but would ultimately be too late to save them. Grisha was then presumed dead for the next five years, with most unaware that he had in fact survived.

Three of the attacking Eldian warriors, Bertolt, Reiner, and Annie then posed as ordinary refugees as they sneaked into Wall Rose to await the next stage of their mission. They would eventually sign on with the military, their comrades totally unaware of their part in the attack.

Shaken by how horribly unprepared they were for the attack, the military bolstered its defenses in the event of further Titan incursions, as the gates of Wall Rose were heavily fortified to slow any Titan advance into human territory should another breach take place. Although horribly outmatched, the military reorganized itself and prepared for the crisis that awaited, as hundreds more Titans now came to inhabit the land once safeguarded by Wall Maria.

Within a year of the disaster, a massive operation to retake Wall Maria was initiated, during which the military conscripted nearly 250,000 citizens, many of them elderly and untrained to wipe out the Titans and secure their lost territories. This campaign proved a major catastrophe as nearly the entire force was eventually destroyed, with barely a hundred survivors living to tell the tale.

The Survey Corps eventually came under the command of Erwin Smith, and have fought a prolonged campaign to wrestle back control of the land between Wall Maria and Wall Rose, with limited success.

The tragedy at Shiganshina would be felt for years to come. Eren Yeager swore revenge on the Titan menace, and began to relentlessly drill himself for the fight ahead. Joining him were Mikasa and his best friend Armin Arlert, another survivor of the carnage at Shiganshina. The three would spend the next few years training to join the Garrison, with Eren hellbent on joining the Survey Corps to take the fight directly to the Titans. All three would eventually come to be major players in the renewed effort to ensure humanity's survival against the Titans.