The Eternal Champion by Michael Moorcock

An ordinary man is torn from his modern life and summoned across the multiverse to serve as a reincarnated champion in a brutal war between humanity and an ancient, otherworldly race. As memories of countless past lives return, he struggles with the crushing burden of destiny, questions the justice of the cause he’s meant to serve, and falls in love with a woman from the enemy side. His ultimate, devastating choice upends his role as savior, exposing the moral ambiguity of heroism and the tragic cost of cosmic balance.