One Piece. Black Clover, though I dropped it when the war arc with elves started. Akami ga Kill if it's a shounen. Fairy Tail had multiple war arcs I think? But there were no losses whatsoever. Magi had war arc. I don't rememebr about losses, but I think the consequences were dire? Didn't Sindbad conquer the world after winning it?
There no loss when alibaba was in coma compared to the old world when aladin mother died and their world too, while all the "guardians/leaders" became djinn
No loss, but the consequences were that the world is now ruled by Sindbad, who turned out to be a bad guy. He mindcontrolled the princess, prosecuted and pressured her country, and so on.