WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 28, "The Dawn of Humanity," now streaming on Funimation, Hulu and Crunchyroll.
Attack on Titan has finally shown the Rumbling from Eren Jaeger's perspective, and the flashbacks of what led him to destroy the world are quite revealing. In one of the more surprising reveals, it turned out Floch Forster was not the only person Eren told ahead of time about his plan to flatten the Earth. Historia Reiss has also known about Eren's genocidal plot from the get-go, and though she was not supportive of it and even vowed to try to stop him, it's clear she never told anyone about what Eren planned to do. So, why would Eren tell the quote-unquote "worst girl in the world" about his plan? And why didn't she tell anyone?
As a descendant of the Fritz Royal Bloodline, Historia would be able to wield the full power of the Founding Titan were she ever to obtain it. As the Queen of Paradis Island, the war with Marley has put her in considerable personal danger. Those who wield the power of the Nine Titans are doomed to die after 13 years, and the Final Season began with Zeke Jaeger approaching the end of his term with less than a year to live. Eren has proven in the past that he can wield the power of the Founding Titan if he touches a Titan with royal blood, so the Paradis Island military devised a plan to force Historia to become a Titan and feed Zeke to her.
If Historia were to become the Beast Titan, Eren would be able to use the power of the Founding Titan by making contact with her -- someone Paradis Island trusts far more than Zeke. The other part of the military's plan for Historia was to turn her into a baby factory, pumping out as many royal heirs as possible so that when Eren Jaeger's term ends, there will be fresh royal blood to inherit the Founding Titan from him. Eren refused to turn Historia and her hypothetical offspring into cattle when the citizens of Paradis Island already live like livestock anyway (in Eren's opinion). This is the reason why he visited her before the Scouts' trip to Marley, to warn her that whenever Zeke arrived on the island, Historia was going to be forced to become a Titan.
While Historia was willing to submit to the military's plans for her to protect Paradis Island, Eren was determined to end the violence forever. The military's plan would allow Paradis to survive, but it wouldn't end the cycle. Eren tried to talk Historia out of her decision to become a baby factory by telling her it was unnecessary, because he was going to destroy the world so Paradis Island could be free. Historia wanted to stop his awful plan, but Eren reminded her that she is the "worst girl in the world," referencing how she once had the opportunity to save Paradis by becoming a Titan and eating Eren but refused to do so and instead freed him to keep the Founding Titan away from the Reiss family.
Eren's reminder to Historia that in the past she's acted selfishly and left the fate of Paradis Island in his hands is likely why she kept his plan to herself. Historia suggested to him that she could get pregnant -- and eventually did -- but instead of the military's plan for her to produce heirs, she did it so that Zeke wouldn't be fed to her the moment he stepped foot on Paradis Island. Historia did the only thing she could do to stop both Eren's and the military's plan, which was to stall in hopes that another solution would be found in the interim. While her pregnancy did thwart the military's desire to turn her into a Titan and feed Zeke to her, it did not allow for an alternative solution to Eren's plan to be found and the Rumbling was activated anyway.
A popular theory is that Historia is pregnant with Eren's baby but given that Eren was against the idea of breeding like cattle in the first place -- along with the feelings he appeared to be harboring for Mikasa -- it's highly unlikely this is the case. Despite her minimal screen time in Attack on Titan's Final Season, Historia has played a pivotal role in the war between Paradis and Marley. Refusing to sacrifice her, more on principle than concern for her specifically, Eren's glimpse of the future after he kissed her hand in Season 3 has allowed him to outmaneuver everyone who has tried to stop him from handling things his own way.
