Michael Goldfarb is absolutely right about something:

 I’ve always enjoyed counterfactual histories. John Keegan, David McCullough, and James McPherson, among others, put out a great collection of counterfactual essays examining the great turning points in military history called What If? They explore what might have happened if the Germans had repelled the allied assault on D-Day, or if Augustus’s legions had conquered Germania.

Bang-on. I’ve only read the first What If? volume, but it’s a classic. In particular, the essay about a Mongol conquest of Europe is fascinating — among other things, it reveals that the Mongols used to place dignitaries of conquered cities into burlap sacks and order their horsemen to stomp the deposed rulers "into paste." The burlap sack was a sign of respect. I keep telling myself I need to read the subsequent volumes and just never seem to find the time.