This edited volume focuses on the use of ‘necessary condition counterfactuals’ in explaining two key events in twentieth century history, the origins of the First World War and the end of the Cold War. Containing essays by leading figures in the field, this book analyzes the causal logics of necessary and sufficient conditions, demonstrates the variety of different ways in which necessary condition counterfactuals are used to explain the causes of individual events, and identifies errors commonly made in applying this form of causal logic to individual events. It includes discussions of causal chains, contingency, critical junctures, and ‘powder keg’ explanations, and the role of necessary conditions in each.