Science fiction is old enough now to have developed a sort of taxonomy. All its subgenres go back to some kind of "What if?" question. What if history had taken a different turn—such as the Confederates seizing Little Round Top, winning the Battle of Gettysburg and going on to win the Civil War? That's "alternate history," as exemplified by Ward Moore's "Bring the Jubilee" (1953). What if time travelers could go back, on purpose, to make history take a different turn? Operatives of opposing sides would start contending to influence the course of events—a "change war" scenario. (See Fritz Leiber's...