The study of causes is fundamental to the historian’s craft. Historical explanations involve asking “Why?” In their interpretations, historians must choose the time frame within which to study the causes of an event. After first finding immediate causes, historians can look for longer-term ones. Choice of a broad time frame may demonstrate that the event being studied was shaped and influenced by longer-term processes than were first perceived. Finding a multiplicity of causes, the historian will need to order them in terms of priority. This chapter shows that while most historians of the Hiroshima decision dwell on Truman and the last months of the war, other historians have found longer term causes compelling. Truman, it is argued, inherited Roosevelt’s policies and was driven by the momentum behind them, especially the longstanding readiness to use the bomb and adherence to the unconditional surrender policy.