What role does oil play in international security? While the threat of “resource wars” over possession of oil reserves is often exaggerated, the sum total of the political effects generated by the oil industry makes it a leading cause of war. Between one-quarter and one-half of interstate wars since 1973 have been connected to one or more oil-related causal mechanisms. Eight distinct mechanisms exist: resource wars, in which states try to acquire oil reserves by force; petro-aggression, whereby oil facilitates domestic political control of aggressive leaders such as Saddam Hussein or Ayatollah Ruhollah Khomeini; externalization of civil wars in petrostates; financing for insurgencies, such as Iranian oil money to Hezbollah; conflicts over potential oil-market domination, such as the United States’ conflict with Iraq over Kuwait in 1991; control over transit routes, such as shipping lanes and pipelines; oil-related grievances, whereby the presence of foreign workers in petrostates helps extremist groups such as al-Qaida recruit locals; and as an obstacle to multilateral cooperation, such as when an importer curries favor with a petrostate to prevent multilateral cooperation on security issues. Understanding these mechanisms can help policymakers design grand strategy and allocate military resources.