Cass Sunstein (Harvard) delves into the philosophy of history, particularly counterfactuals and causation, with his review in The New Republic of Altered Pasts: Counterfactuals in History by Richard J. Evans. (via Philosophy Matters)

Historians cannot conduct randomized controlled trials, because history is run only once. Yet they nonetheless develop hypotheses, and they attempt to evaluate them by reference to the evidence. Evans is himself engaged in this enterprise. There is no difference between hypothesis-testing and counterfactual inferences. Any claim of causation, resulting from such tests, requires a statement that without the cause, the effect would 
not have occurred.

Not all metaphysicians would agree.