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.

During their travel from the borderline of Paradis to the Walls, Reiner was nearly eaten by a Titan. Marcel sacrificed himself to save Reiner, who watched helplessly as their friend was consumed. As they continued on their mission, the Titan absorbed Marcel's Titan power and returned to its human form as Ymir, an Eldian girl who had been turned into a Titan around sixty years before. With her humanity regained, Ymir made her way to the Walls to live a new life.

On the morning before the fall, Commander Keith Shadis returned to Shiganshina District from an expedition beyond Wall Maria in which less than twenty survivors returned from a force that had numbered in over one hundred. In humiliation and defeat, Commander Shadis handed over his position to Squad Leader Erwin Smith. With the Survey Corps' drastically reduced numbers, the people of the Walls were all the more susceptible to the Titans.

Elsewhere in Shiganshina, Grisha Yeager, a holder Titan power for 13 years, made plans to travel to the interior in order to finally retrieve the Founding Titan from the Reiss family, the true royals within the Walls. Before leaving his home, he promised his son Eren Yeager that upon his return, he would show him the secret he had kept secret in the basement of their home.

Shiganshina District
In the afternoon following the return of the defeated Survey Corps, the attack on the Walls began when Bertolt Hoover as the Colossus Titan appeared beyond the south gate of Shiganshina District, breaking the gate apart and breaching Wall Maria in a single kick. Presumably at this time, Annie Leonhart used the Female Titan to attract countless Titans to the breach where they would invade humanity's territory. As Titans began to enter the district, the civilians started to evacuate in a horrified panic while the Garrison set up a hasty defense.

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 Garrison soldier Hannes, a family friend, rushed to their aid. Confident he could slay an incoming Titan, Hannes rushed into action hoping to save the entire family, but soon found himself facing the Titan, who unbeknownst to the group was Dina Yeager, Grisha Yeager's first wife that had been turned into a Titan thirteen years ago. Unnerved and losing the will to fight, Hannes saved Eren and Mikasa and evacuated with them as Carla, accepting her fate, was eaten by Dina's Titan as Eren watched.

Unable to contain the invading the Titans, the Garrison evacuated any survivors they could to the gate and packed them into ferries that would transport them to Wall Rose. A small group of soldiers under the command of Hugo assigned cannons to defend the gate, trying to hold off the advancing Titans. The soldiers debated closing the gate, which would stop the advance of the Titans but also doom the civilians who had not yet evacuated. Before a conclusion could be made, Reiner Braun as the Armored Titan charged the north gate of Shiganshina, with Garrison cannonfire proving useless against him. The Armored Titan succeeded in breaching the north gate, and with that Wall Maria was fallen.

The Reiss family massacre
In the interior, news of Shiganshina's fall had reached Grisha Yeager, who made his way to the Reiss Chapel where the family had gathered in response to the crisis. After frantic pleas which resulted in nothing, Grisha battled Frieda Reiss, queen of the Walls, and defeated her. He ate Frieda out of her Titan's nape, stealing the Founding Titan, and afterwards slaughtered the members of the Reiss family within the underground of the chapel. Only Rod Reiss, the head of the family, escaped from the slaughter, and he began to seek out his illegitimate child Historia Reiss out of the need for another royal-blooded individual to reclaim the Founding Titan. After destroying the Reiss chapel, Grisha made his way south to where the refugees from Shiganshina District would arrive.

When the refugees from Wall Maria entered the safety of Wall Rose, Grisha had arrived at the scene to search for his family. He came across Mikasa and Eren, the latter of which told him of his mother's death. Ex-commander Keith Shadis who was nearby was distraught at the news, and Grisha led his son into a nearby forest, urging Keith to stay uninvolved. Within the forest, Grisha urged Eren to return to Shiganshina in the future in order to discover the secrets within the basement of their home. He injected Eren with a Titan serum stolen from the Reiss family, turning his son into a Titan. Eren's Titan consumed Grisha, inheriting his Titan power and the unusable Founding Titan with it. Eren fell into a state of shock, falling unconscious, and Keith Shadis found him shortly afterwards and returned him to the refugee center.

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.

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.