What role did oil play in the decision to invade Iraq in 2003? We still do not know exactly why the Bush administration went to war against Iraq, and we may never know. Certainly, no compelling evidence, either in the form of declassified documents or participants’ memoirs, has yet emerged indicating that oil was a prominent factor or constant consideration in the thinking of decisionmakers within the Bush administration. But oil is nevertheless critical to understanding the decision to invade Iraq and remove Saddam Hussein from power. Oil did not make a U.S. war against Iraq inevitable. But it did much to set the stage for war, greatly increasing the incentives to topple Saddam, by any means possible. Indeed, it is hard to imagine the invasion of Iraq ever having occurred but for oil. Only by understanding the ways in which oil has infused the U.S. strategic calculus in the Persian Gulf can we fully make sense of the war.