Read this story many years ago so my memory may be a bit fuzzy on it.
1. Current year man is abducted by White balls of light.
2. Taken to future Earth period where he waits with those future humans
3. Space ship arrives from storage planet and he is put on it and travels into the far future.
4. In the far future the then humans in the balls of light are at war with each other white vs green I think are the colors.
5. He must use the Space ship to win the war?
Just remembering these bits and pieces has tweaked my interest in wanting to reread the story, but can't find it now. Or remember the name of it or the author.