Oil, the State, and War: The Foreign Policies of Petrostates, by Emma Ashford, Georgetown University Press, 365 pages, $34.95

It doesn't take a Ph.D. to see that oil drives conflict. Just looking at the recent history of America's interventions in the Middle East will do.

During the Tanker War of the 1980s, the U.S. Navy fought Iran to protect Iraq's oil. Then, during the first Gulf War, the U.S. military fought Iraq to protect Kuwait and Saudi Arabia's oil. After three decades of war in Iraq, the United States came full circle, with the Trump administration threatening Iran while Iran threatened Saudi Arabia's oil.

Less understood is how oil drives conflict. The popular view, espoused both by many anti-war critics and by violence enthusiasts like former President Donald Trump, is that larger countries go to war to steal the oil wealth of smaller ones. Strong consumers take what they can; weak producers provide what they must.

Emma Ashford, a foreign policy scholar at the Stimson Center, makes the opposite claim in Oil, the State, and War. It isn't the need for cheap energy that drives foreign policy, she argues; it's the economics of energy production that make petrostates more trigger-happy. On one hand, control over energy markets removes constraints on warmaking. On the other hand, the "resource curse" warps political institutions. And of course, oil money helps governments buy fancy weapons.

Oil is unique in how it influences state behavior. Like many other natural resources, petroleum is scarce and expensive. Unlike those other resources, oil is necessary for the world economy to keep running. And all oil is bought and sold on the same global market, priced in U.S. dollars, meaning a change anywhere affects prices everywhere.