Eren Jaeger fought alongside the Survey Corps to reclaim their city stronghold from the ravenous Titans throughout the first three seasons. While they could push back against the monsters, they eventually discovered that Paradis Island — thought to be home to the last remaining humans — was effectively a prison for people who could turn into Titans. Humanity was alive and well in the nation of Marley, and Eren’s people were caught in one giant internment camp.
The fourth season saw a broken Eren arrive in Marley years after the revelation. The Eldians of Paradis Island began an invasion of the outside world, using Eren’s growing Titan abilities as a lynchpin.
While both sides attempted to achieve a form of unity, they had difficulty seeing eye to eye. Eren proved to be incredibly chaotic throughout their interactions, appearing to sympathize with violent Eldian radicals.
The first part ended with Eren ready to take on the Marleyan forces.