Counterfactual reasoning is a practice that is very frequently resorted to in everyday life as well as in serious analysis of historical outcomes. However widespread and indeed indispensable, counterfactual analysis is recognized to be a very weak, problematical method because of its conjecture of possible alternative actions. More disciplined uses of counterfactual reasoning evaluate historical events carefully to distinguish highly plausible from implausible or barely plausible claims for alternative historical outcomes.
Counterfactuals are a way of rewriting history (exploring the possibility of an alternative outcome) by conducting a mental experiment -- i.e., "if only this rather than that had been done, the outcome would have been quite different." Some counterfactual assertions are more plausible than others.