WARNING: The following contains spoilers up to Season 4, Episode 10 of Attack on Titan, "A Sound Argument," now streaming on Crunchyroll, Funimation, Amazon Prime and Hulu.
In Attack on Titan Season 1, Armin and Eren wanted to know what was beyond the walls; however, as the anime progressed, they realized not only was the world much larger than they thought, but they also realized Eldians were seen as devils to the rest of humanity. Furthermore, they learned Titans were formerly Eldians, and Marley sent their Warrior Unit -- Annie, Bertholdt, Reiner and Marcel -- to steal the Founding Titan.
With those living in the walls being completely in the dark for decades, all this came as a surprise. While the Eldians on Paradis just learned about the outside world, Marley pushed anti-Eldian propaganda, so the truth behind their ongoing conflict had long been unclear; however, Willy Tybur revealed the truth about their history in Season 4, Episode 5, "A Declaration of War."
Why Marley Believed Eldians Were Devils
Even before the Survey Corps learned about Marley, it was alluded to that those not from within the walls believed the Eldians of Paradis were evil, with Bertholdt calling his former comrades "spawn of the devil." Initially this could've been seen as the Warrior reacting to Armin's lie about torturing Annie; however, now knowing the history Marley was teaching its subjects, it becomes clear why he chose those words.
According to Marley's history books, around 1800 years ago, Ymir Fritz made a deal with the devil to obtain the Power of the Titans, and when she died, this power was split into nine Titan shifters, who created the Eldian Empire. They defeated Marley, ruled the continent and oppressed those who weren't Eldian, leading to centuries of genocide.
It wasn't until the Great Titan War that Marley saw the opportunity to end the Empire. The Marleyan hero Helos, along with the Tybur family, tricked the Eldian Empire to turn on its own and forced King Fritz, the Founding Titan, to flee to Paradis with some of his subjects. However, Fritz still had power, and millions of Titans wait for the Founding Titan's command to trample humanity, hence why the Warrior Unit was sent to Paradis.
Why the Eldian Restorationists Believed Ymir Was a Savior
While this history was widely believed by those on the mainland, including the Eldians in the internment zones, some questioned this. The Eldian Restorationists, which included Eren and Zeke's father, Grisha, were given documents by Eren Kruger and came up with a different story, as revealed in Season 3, Episode 20, "That Day."
While they could not understand the language the documents were in, based on the images and their faith, they believed that Ymir awakened the Power of the Titans to improve agriculture and infrastructure, helping the continent and people prosper. They also realized King Fritz took the Founding Titan to Paradis because he refused to fight, abandoning many of his people on the mainland to deal with the repercussions, which included many Eldians being permanently turned into Titans to combat Marley's enemies.
What Actually Happened Between Marley and Eldia
Because the War Hammer Titan has been passed down within the Tybur family, they inherited the memories of their ancestors; therefore, they know the truth behind this conflict. As Willy shared with the world, Helos and the Tyburs did not end the Great Titan War; King Fritz did. He was horrified by the Eldian Empire's history and, "pained by the endless oppression of Marley."
Once he inherited the Founding Titan, he teamed up with the Tyburs to fabricate a hero, Helos. Fritz then moved as many of his people as he could to Paradis, with the remains Eldian Empire collapsing in his absence. On Paradis, he closed the walls to the world, which were made of Titans, and swore that if anyone threatened their peace, he would use them to trample humanity. This was another lie to keep others at bay. He then erased the memories of those within the walls of the outside world and renounced war, passing down this vow to his family members who inherited the Founding Titan.
He was also prepared to accept Paradis' downfall if Marley decided to attack the island. The sins of the Eldian Empire could never be forgiven in his eyes, and he was fine with this, even if it meant only enjoying a brief moment of peace within the walls. For peace, the Tyburs and Fritz sold out all Eldians to Marley, leading to their current oppression. While the history between Marley and Eldia has been manipulated many times, in this new chapter, no one is safe, as Zeke and Eren's plans have led to an all out war where many lives have already been lost.