An interesting alternate history essay from the New York Times. What if World War I had never happened? What if John F. Kennedy hadn't been assassinated? Fascination for such scenarios is growing, with a handful of books on that theme to be published this fall. Yet such counterfactual, or virtual, history, as it is often called, has long intrigued historians, filmmakers and others with its suggestion that destiny hangs on a single event, or even a triviality. In What If. . . ?, a 1982 collection of historical fictions edited by Nelson W. Polsby that is now out of print, a handful of scholars challenge the tendency to take the past and present for granted, imagining what would have happened if, for instance, Hitler had gotten the atomic bomb. The political scientist William H. Riker, who di