Paradisians want a strong leader who can stand up to Marley. They're finding themselves in a situation as complicated as the Uprising Arc, but without a leader who can play the game as well as the last time.
Leadership isn't just a role; it's a mindset. A mindset that Hange and Armin can't fit into.
The military turn against Eren as they plan to have another soldier steal his Founding Titan. Back then, when the government wanted to kill him, Erwin protected him and utilized his power for the greater good. Now, Hange's long-term imprisonment of Eren is the cause of commotion that splits the Eldians into different fractions.
Unfortunately, a lot of AOT fans believe the narrative of "nothing could have been done" - which is a simple cop-out of the situation. A LOT could have been done that the characters did not do. The political landscape presented in the manga isn't an impasse, which is the convenient narrative that Isayama wanted us to believe. Considering how quickly and easily the Marleyans cooperated with the Walldians in the final chapters, that "impasse" narrative wasn't accurate. Even a hard-headed leader like Magath bonded with Shadis and admitted he was wrong. So imagine Paradis could make friends with several countries if Marley wasn't interfering.
I find it astounding that Paradis was sitting on a weapon of mass destruction and never bothered to actively use it as a deterrent and a bargain tool to push for aggressive diplomacy and fracture the collective hatred that the world has for Paradis. I think that someone like Erwin, who has consistently shown the most proactive, pragmatic and idealistic visions for his decisions, would have been able to achieve a realistic political maneuver to save Paradis without resorting to a full Rumble.
Hange: looks at cars and thinks they can eat carrots. How can you expect her to "read" the enemies?! By locking up dissenters, she makes enemies of her soldiers. She admits in hindsight that bad leadership has brought a civil war upon the land.
Hange's leadership sharply contrasts Erwin's: Remember when Eren shouted in Court and horrified everyone? Remember when he was brought back from the courtroom, Erwin sat down, shook his hand and said, "You have my respect. Let's work together, shall we?"
[ Erwin was able to use people to the best of their abilities. He respected Armin's intellect and told his soldiers to listen to Armin while no one else would. He was also able to "read" Levi and completely "reformed" him from a thug to a hero. Although he had never met Levi before, he was able to "read" him correctly and used his emotions for Farlan and Isabel to capture them. And when Levi broke down after they died, he knew that Levi was about to regret it and thus he gave him a "no regrets" speech.
He's the mastermind of >90% of the strategies in Seasons 1-3. He is the kind of man who won't force anyone into obedience but gives them options. He knows how to gain trust and use people. He overthrew the government with minimal resources and allies. Those skills would be invaluable when dealing with other nations.
If Erwin were alive, he would make agreements of peaceful resolution with the world. There is a reason he's been removed from the story just before the Basement Reveal just like there is a reason Levi was removed just before the finale: These guys excel at their respective strengths way too much.
You could simply use normal diplomacy with the Rumble as a negotiation chip, exploit the hatred that other nations have for Marley to shift the global hostility from Paradis to Marley, offer trade using Paradis-exclusive exports like iceburst stones which are an essential energy source as important as gas and oil. As long as Paradis crafted a careful public reputation as a nation that could end the world anytime, no nation would attack them because no nation is stupid enough to tackle someone with "nuclear weapons" and whose economy is so intertwined with the world. Also remember: Marley's true objective is Paradis' resources and the Founding Titan; fear and history are just a propaganda to pull the word to its side. Aggressive negotiations and manipulations from Paradis will absolutely fracture this "common hatred" that the world has for Paradis because GREED drives world politics.
I am not so naive as to suggest that somebody can miraculously make their enemies turn over new leaves and live in harmony. IF you think that's what I've been saying, you're missing the point:
The reason we don't have WW3 is because war is too costly, not because we like one another or we are a peaceful species. Diplomacy isn't genuine friendship; it is negotiation-manipulation.
Paradis would ask other countries for cooperation under the premise of mutual economic development and cultural exchange. Again, Greed drives the world. Also, I imagine that Paradis would be able to make allies with several countries that are Marley's enemies by simply helping them win the war against Marley. Losing the 4-year war also means Marley wouldn't be able to expand their influence.
Paradis' goal shouldn't be "everlasting peace" because I doubt humanity can ever achieve that. Their priority would be to destroy the enemies' military forces, and at the same time, interweave their economy and technology with the rest of the world. The goal is to become a superpower and remain as such, which they can totally do without a full Rumble.
Erwin would speak with other countries' leaders, and things would turn out very differently. If any Paradisian can earn respect from their enemies, it is Erwin. His persuasion skills would come into play; he'd likely succeed in making many nations go along with his plans. Let's not forget that Marley has lots of enemies around the world, too. Paradis would appear worldwide as the liberator of the peoples oppressed by Marley and leader of the free world.
"I guess Isyama had to nerf Paradis before the Marley Arc so they wouldn't immediately curb stomp the shit out of Marley. Imagine Paradis with a Colossal Erwin, a fully-realized Founding Titan, a not-so-depressed Levi, Mikasa, Hange, the list goes on."
Eventually, as Titan powers disappear or become obsolete, no one would continue fearing Eldians. Marley will no longer be able to use Titans as war weapons so they'll no longer have a reason to invade. Paradis would at least be on equal footing with the world now. Years of mutual, interdependent developments will erase that hatred/fear. Real-life politics is proof that a foreign power's treatment of the people will determine whether that power will be liked or disliked.
Internal affairs:
Erwin once again would be reforming the Paradis government, as Zachary became more like the monarchs they had deposed of.
He would be the one who would actually listen and talk to Eren. Eren believed that none of his friends could handle the truth so he kept all information to himself. However, it's different with Erwin because he could handle the truth and would go about it in a different way; he would have made rapid decisions that would satisfy him.
Dealing with hostility isn't something new to him as for more than half of his life, he was part of the hated Survey Corps.
He would 100% prevent the wine poisoning.
He would raise the soldiers' morale and unite the military under one goal.
If Erwin was alive, you would see his amazing interactions with the outside world that was full of philosophical nuances, along with decisive "battle of minds" political games.
It is said that "the truth might be completely different from what Kruger, Grisha and everyone know". Given his curiosity, Erwin would definitely continue to get to the bottom of this truth.
Watching Paradis dealing with the outside world without a full Rumble would have been incredibly interesting. Isayama could have woven a really intriguing, difficult story out of that concept.
Read another fan's viewpoint on what Erwin would do.
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