Last week, Bryan Caplan raised the issue of what would have happened to the world had Lenin died five years earlier. One of the commenters criticized the idea of considering counterfactuals and, in response, I defended the idea of counterfactuals.
One that I have struggled with, given my belief that the U.S. government should stay out of other people's wars, is what would have happened had the U.S. government not got into World War II. Before Pearl Harbor, a strong majority of Americans wanted to avoid the war. World War II was really the turning point that made the U.S. the world's superpower and the world's policeman. From that viewpoint, therefore, even aside from the huge cost of WWII to the U.S. at the time, I think it would have been a good idea to stay out. Staying out would have been easy. Had FDR not tried to strangle Japan's economy by trying to cut off its supply of oil, Japan's government would probably not have attacked Pearl Harbor. And without that attack, FDR would not have been able to get into the war with Germany.
But most people I talk to say that, even though the U.S. could have stayed out of World War II, it shouldn't have.
That's too big an issue to resolve here, but here's the point I want to make. When I discuss this issue with those who favored U.S. participation in WWII, invariably we engage in counterfactuals. Would Hitler have conquered the Soviet Union? What would have happened to Western Europe? What would have happened to China? Etc. Ken Judd, a colleague at Hoover who favored U.S. participation in WWII told me, when we were discussing this a year ago, that I needed to have good answers to these questions in order to argue for U.S. staying out. But I pointed out to him that it's symmetric: he needed to argue persuasively for different answers to these questions in order to support U.S. participation. In other words, even those who want to argue for the war must argue that what would have happened without U.S. participation would have been worse.
Bottom line: to argue for anything, there's no avoiding counterfactuals. And it's symmetric. Those who want to make a case for U.S. participation in WWII can't avoid the need to figure out what would have happened without U.S. participation.