The squad treated the three cups as proof of an unknown third party because, while they knew Ymir had left with Reiner and Bertholdt, her loyalty was still seen as unclear and conditional rather than fully aligned with the enemy. She had sided with them mainly out of concern for Historia and had previously shown she could act independently or even against them, so the Scouts had no solid reason to assume she was still camping with the pair as an active member of their group by the time of the Shiganshina operation. The cups (containing the foreign black liquid later revealed as coffee) indicated three people had recently used that specific campsite, pointing to Reiner, Bertholdt, and one additional outsider they hadn’t accounted for—which correctly turned out to be Zeke. Assuming the third cup belonged to a completely unknown person was the more cautious and logical conclusion based on the limited information available at the time, rather than presuming Ymir’s continued presence without confirmation.

The scene you’re pointing to is the one where Eren drops the “Ackerman host” story on Mikasa and Armin, Armin swings at him, and Mikasa immediately pins Armin down. That moment looks robotic because she’s reacting on pure instinct in a high-stress confrontation involving the person she’s protected since childhood—the same way she’s always moved faster than thought when Eren is threatened. It’s the most dramatic example of that reflex, which is why it sells Eren’s claim so well in the moment. But that claim was a deliberate lie. Zeke later confirms to Eren that there’s no evidence of any genetic “host” programming in the Ackerman line; the headaches and the extreme protectiveness come from her genuine feelings and the trauma that bonded them, not some forced bloodline slave bond. Isayama himself has said the Ackermans’ loyalty is their nature, not a compulsion that overrides free will. The proof is in what happens afterward: Mikasa is able to reject Eren’s path, fight against him, and ultimately choose to stop him. If she were truly a robot programmed to protect him no matter what, that choice wouldn’t have been possible. The instinct in that scene is real—it’s just love and years of protective habit, not the genetic mind control Eren invented to push her away.

It wasn’t really “taking over” in the traditional sense of an invading army with strategy or logistics. The Pure Titans that flooded the outer territories (and later the walls) are mindless, driven by a single biological imperative: devour a human who possesses the power of the Titans so they can return to human form. They don’t need supply lines, leadership, or even self-preservation beyond that instinct, and once the Colossal Titan kicked open the gate at Shiganshina, the sheer numbers and the fact that the people inside had spent a century living under the false belief that they were the last of humanity made resistance collapse fast. The military was structured around static defense of the walls, not mobile warfare against an endless horde that regenerates and feels no pain or fear. As for betrayal from within—yes, in multiple layered ways, though not the kind of sudden coup most people picture. The original “founding” of the walls involved the First King using the Founding Titan to erase memories and enforce isolation, and later the Warrior unit from outside deliberately infiltrated the military ranks years in advance. They weren’t random turncoats; they were children raised with the mission to retrieve the Founding Titan and break the walls. The ease of the initial breach and the subsequent chaos came from that combination of prepared sabotage plus a population that had been deliberately kept ignorant of the outside world and the true nature of the Titans.

Pure Titans aren’t actually hungry in any biological sense—they don’t need food to survive, don’t digest what they swallow, and gain no energy from it. Their drive to eat humans comes from a hardwired instinct created when an Eldian is injected with Titan spinal fluid and loses their mind: they endlessly seek out a Titan Shifter (one of the Nine), because consuming one is the only way a Pure Titan can potentially return to human form. That’s why they zero in on people and ignore almost everything else. When no humans are around they sometimes turn on each other out of the same compulsive urge or simple mindless aggression, but other Titans aren’t the real target. The whole behavior is ultimately tied to the deeper mechanics of the Founding Titan and the Paths, which is why the “why do they eat people?” question stays interesting even after you learn the surface-level answer.

You’re not the only one—this detail gets noticed on almost every rewatch and immediately sparks the same question. In that scene from the 57th Expedition, Reiner has just cut himself free from the Female Titan’s hand after she “caught” him; the steam is coming from Annie’s Titan blood evaporating off his body. Separated Titan blood is extremely hot and steams away almost instantly, which is why it looks exactly like regeneration. The visual is so similar to how shifter healing works that it functions as perfect foreshadowing either way. Whether it’s purely Annie’s blood or a subtle nod to Reiner’s own abilities (he can even suppress regeneration to stay undercover), the animators used the ambiguity deliberately. Combined with the other small hints scattered through Season 1, it makes the later Armored Titan reveal land even harder once you know what you’re looking at.

When Bertholdt transformed into the Colossal Titan and kicked open the gates of Wall Maria at Shiganshina in 845, the Wall Titans were still sealed and dormant inside the Walls themselves. King Karl Fritz had used the Founding Titan roughly a century earlier to create and harden tens of millions of these Abnormal Titans (which resemble smaller Colossals) into the concentric structures of Walls Maria, Rose, and Sina, leaving them immobile due to the lack of sunlight and the royal vow that kept them in place as a passive deterrent. They played no role in the initial breach because only the Founding Titan’s power could unharden the Walls and command them to march—something that didn’t happen until Eren activated the Rumbling years later. The gates were simply the vulnerable entry points the Warriors exploited with the Colossal and Armored Titans, allowing regular Pure Titans from outside to pour in while the Wall Titans stayed embedded and inactive throughout the fall of Shiganshina and the later Trost incident.

Eren didn’t spare Bertholdt because he couldn’t—or rather, he actively refused to. Through the Attack Titan’s ability to peer into and influence past events via the Paths, future Eren specifically directed Dina’s Pure Titan to bypass Bertholdt and devour Carla instead. This wasn’t some oversight or pure sadism; it was a deliberate, timeline-locking choice. If Dina had eaten Bertholdt, she would have inherited the Colossal Titan while still possessing royal blood. That would have given Grisha (and later Eren) an earlier, cleaner route to the Founding Titan’s full power without needing to orchestrate the entire 13-year cycle of hatred, the fall of Wall Maria, and the specific chain of events that put Armin in position to eat Bertholdt later. The future Eren had already seen—and accepted—would have collapsed. The tragedy is that Eren did choose his mother’s death. In the Paths, he admits he directed Dina that way to keep the exact sequence intact so his friends could ultimately stop him and survive. It wasn’t that he “wanted” Carla dead out of malice; it was that he had already locked himself into a future where her death was the necessary fuel for his rage, the Rumbling, and the only outcome where Paradis and his friends weren’t wiped out. He traded his own mother’s life for a future he had already glimpsed and refused to abandon, even knowing it would break him. That’s the full weight of the Founding Titan and Attack Titan’s power working together—Eren wasn’t just following fate; he was actively writing the past to preserve the only ending he could live with.