Daten Sakusen

A story of a immortal man, Under, who is stranded in the middle of a grand war between humans and demons. Under cannot remember where he came from or what he really is, and despite being an undying immortal that can regenerate himself over and over no matter what happens, he lives his like a corpse, with no purpose or desire.

That all changed when he got captured by the Demon army and was put on the "Space Execution" with Recobell, where the two were locked up on an hot air balloon and sent up the atmosphere to die. Through his conversations with Recobell, Under slowly develops motivations to live, and seeks to find the meaning of this seemingly endless and pointless war between the two factions.

(Source: ruggia's (raw) manga rants blog)