Im gonna suggest two slightly different anime from everyone else. Black Lagoon and Saga of Tanya the evil.

Black Lagoon is basically a show about some mercs in south east asia during the 90s, and the weird shit they get up to also it can be rather melancholy at times.

Saga of Tanya the evil is about some poor bastard who picks a fight with a god and promptly gets reincarnated as a little girl in WW1 Germany, where they proceed to succeed downwards into a frontline role when they just want to be a rear line officer. Also they commit technically not a warcrimes.