The Lord of the Sands of Time (novel)

Sixty-two years after human life on Earth was annihilated by rampaging alien invaders, the enigmatic Messenger O is sent back in time with a mission to unite humanity of past eras--during the Second World War, in ancient Japan, and at the dawn of humanity--to defeat the invasion before it begins. However, in a future shredded by love and genocide, love waits for O. Will O save humanity only to doom himself?