A civilization builds huge AI-driven mecha to defend against an existential alien threat and wins. The mecha retire, going into a low-power mode where their AIs are dimly awake, but the vehicles themselves are immobile.
Many years later, the civilization has fragmented into smaller states at war with one another. Ambitious fighters from these states go to wake up the war machines, but find that the AIs have diverged wildly in personality over the years. Some want to remain in their peaceful dormant state. Some are curious about the current state of the world, but uninterested in fighting again. Others want to fight, but don't relish the idea of fighting humans or other mecha. And some are eager to get back into battle, whatever the nature of the fight.
I picture an aesthetic like Nausicaa of the Valley of Wind plus a bit of Shadow of the Colossus. Vehicles two or three stories tall, clad in ceramic armor rather than metal, in organic shapes resembling insects or crustaceans, at rest in places of serene natural beauty.
And Sense8 is so worth a watch.