HEADCANON: During the apocalypse, the titans attacked and Ludwig lost his memory again. But since Prussia wasn't there with him to remind him that he was a country, he assumed he was human. He took on the name of Erwin Smith, and swore to kill all the titans around the world. Levi is really Japan , trying to get Germany to remember his past. That's why neither of them seem to care for humans, and why they will have to watch their friends die around them, wondering why it feels so familiar.