This article addresses the role of counterfactuals in historical and ethnohistorical explanation. Drawing primarily on examples from the Conquest of Mexico, it argues (1) for a useful role for counterfactual analysis, not in writing fiction, but in assessing pivotal causation and proofing causal arguments; (2) for a clearer understanding of causation in historical records, especially of ethnohistorical subjects; and (3) for a way in which this perspective can be employed to argue for solidly grounded revisionist interpretations of events.