WARNING: The following contains spoilers for Attack on Titan Season 4, now streaming on Funimation, Hulu, and Crunchyroll.
Eren's decision to start the Rumbling and flatten everything beyond Paradis Island isn't an easy one. Nor is it a simple plan to enact. Initiating the Rumbling would require time, thought, commitment and sacrifice. Eren, however, had two more things to aid in his path of destruction, the Attack Titan and Historia Reiss.
The Attack Titan has the unique power to see the future when it comes in contact with someone of Eldian royal blood. For Eren, this happens at the end of Attack on Titan Season 3 when he came into contact with Historia Reiss. From there, everything he does is based on things he saw in the future. Here's a look at how this happened, what Eren saw, and how he acted upon it.
It all started at the military ceremony shortly after the climax of the "Return to Shiganshina" Arc. As Queen Historia was bestowing medals upon each of the surviving heroes of the Scouting Regiment, Eren was contemplating what to do about the looming threat of Marley. He needed a plan that wouldn't put Historia's well-being at stake. When the queen reached Eren, the two made contact and Eren had a vision. It showed Eren's father Grisha shortly after the fall of Shinganshina. He was in the Underground Chapel imploring Frieda Reiss, then the King of the Walls, to do something about the Titans before they killed his family. The scene then changed back to Eren with a look of dread on his face.
It's not shown here, but Eren actually saw everything at that moment. He foresaw his attack on Marley, his interactions with Grisha, and his use of the Rumbling. He even saw what happens after the Rumbling is initiated. From this moment on, everything Eren said and did is guided by what he saw in his vision. A clever hint about Eren's intervention in past events was also hinted at here. Most of the flashback was shown from Grisha's perspective, but there was one shot that briefly shows Grisha's face. This mainly confirmed that it's Grisha talking in the flashback, but it had another hidden meaning; it's Eren's point of view when he coaxed his father into killing the Reiss family. This was one of many subtle moments that hinted at the influence Eren had over not just the rest of the series but also many things that came before it.
It's important to note that Eren's contact with Historia only provided the foundation for his plans. At the time, all of these memories came to Eren completely by chance. He also had no way of knowing if the events of his apparently bleak future were set in stone. He still wanted to see if he could find a better solution for gaining freedom for himself, his friends, and the Eldian people.
It wasn't until he experienced certain events in his future that he was certain of its inevitability. These events include Sasha's death and his interaction with Grisha. It was only after he confirmed and resigned himself to this aspect of his destiny that he became steadfast in his resolve to initiate the Rumbling.
Eren's contact with Zeke may have made the Wall Titans move, but it was his contact with Historia that really got the Rumbling started. It was at this point that Eren was given something to strive for and begin moving towards it. Everything Eren has done since this moment and will continue to do will have been in the service of his vision.
