WARNING: The following contains spoilers for Season 4, Episode 10 of Attack on Titan, now streaming on Crunchyroll, FUNimation, Amazon Prime and Hulu.
Attack on Titan's plot and stakes have totally changed since Season 1. At first, Eren Yeager and his friends were determined to save humanity's last city from the mysterious and deadly Titans, but that was all a ruse. Those Titans are people, living weapons sent by the Marley Empire from overseas. This is actually an international war, not a zombie survival scenario.
Why would the Marley Empire do this to the people of Paradis Island? The Marley Empire conquered the tyrannical Eldian Empire a century ago, and the Marleyan rulers are determined to make the Eldian people the most shameful group in existence. But Zeke Yeager isn't having it -- all the Marleyan propaganda has backfired on him, ironically pushing him into the arms of his fellow Eldians in need. Now Zeke is teaming up with his half-brother Eren to take down the Empire once and for all.
The Marley Empire's Endless Propaganda
Near the end of Attack on Titan's third season, viewers got a good look at the world beyond the walls and beyond the sea... and it's not a nice place. Beyond the confines of Paradis Island stands the Marley Empire, with World War I-era technology and all kinds of cruel laws and propaganda keeping Eren's countrymen under the Marleyan boot. Here, the Marleyan people reign supreme, and the Eldians live oppressed within the capital city's internment zone. They can't even go out and about without a distinct yellow armband to mark themselves as Eldian, and Eldians of any age or sex may be tormented or abused at the hands of the Marleyan guards without consequence. The Eldians are the Marley people's plaything, and the Eldians have learned to accept it as fate.
These interred Eldians have a narrow route to upward mobility: becoming a Warrior. Promising boys and girls may undergo Warrior candidate training, such as Zeke Yeager, Annie Leonhart and Gabi, and these trainees may inherit one of the Titans that the Marley Empire possesses. Successful Warriors fight on behalf of the Marley Empire against its enemies, and these Eldians and their immediate families are venerated as honorary Marleyans. This is the world that Grisha Yeager and his son Zeke were born into, and it taught them to fear and hate the remaining Eldians on the faraway, near-mythical Paradis Island. However, Zeke's story shows that the constant "you must hate your fellow Eldians!" propaganda can backfire.
The Propaganda Backfires -- and an Alliance of Brothers Is Born
Most Eldians are brainwashed by the brutal Marleyan propaganda, internalizing it to the point of calling the Paradis Island inhabitants "devils" despite having never met them. For the most part, this brainwashing proves effective, but rebellious Eldians such as Grisha, Zeke and the Owl (Eren Kruger) feel differently. Instead of accepting the "you must hate Eldians" message, these Eldians deeply resent it, leading them to despise the Marleyans for spreading such cruel propaganda. It drives them to cherish and protect their fellow Eldians at all costs, and it can even motivate Grisha, Dina Fritz and others to form an Eldian restorationist movement. It's true that Zeke turned in his parents for this, showing apparent loyalty to Marley, but Zeke isn't a total Marleyan puppet.
Despite turning in Grisha and Dina, Zeke is now using his Beast Titan powers, tactical acumen and influence to rally the people of Paradis Island, particularly his half-brother Eren, into fighting back. Zeke refuses to internalize any self-loathing based on his ethnic identity; instead, the propaganda pushed him in the opposite direction and galvanized his will. Now he wants to protect Paradis Island's Eldians more than ever, and he's ready to use Historia Reiss's royal blood and Eren's Founding Titan as a nuclear option. Once the rumbling is set off, the entire world will fear the Eldians' endless army of colossal Titans and back off at once.
The Eldian restoration movement was a doomed effort in Zeke's eyes. He shut it down to keep the restorationists out of his way, so he can save the Eldian people on his own terms with superior means. It is now clear that the "superior means" involves the rumbling and the deterrent of that Colossus Titan army. And that requires the assistance of Eren Yeager, who houses the Founding Titan. Eren can save Eldia in ways that his and Zeke's father could never have dreamed of, let alone achieved. Thus, the half-brother alliance was born.
Every 13 years, a new pair of Titans (one royal, one non-royal) will do this, and give Paradis Island the time it needs to prepare further defenses. At least, that's what Zeke's plan is like so far, and it doesn't matter what the Marleyans say or think about it. They can't convince Zeke to hate and torment his fellow Eldians any further -- they can only watch as Zeke brings the full fury of his people to bear against them.