Where there's oil, there's often war. Specifically, petrostates -- countries where oil export revenues account for at least 10 percent of GDP -- are at least 50 percent more likely than non-petrostates to engage in interstate conflicts. And in those conflicts, they are almost twice as likely to be the attacker.
The reasons for this are the topic of the book Petro-Agression: When Oil Causes War by Jeff Colgan of American University. According to Colgan, petrostates aren't move violent because of competition over oil resources. Rather, their propensity for war has to do with the way oil interacts with a state's domestic politics. Just as many have argued that oil decreases the quality of governance by removing incentives for leaders to build responsive state institutions, Colgan argues that oil-fueled leaders see lower costs for instigating conflict.
Take for instance, the Iran-Iraq war. Despite the fact that domestic opposition to continuing the war was growing in 1981-1982, Ayatollah Khomeini chose to press on. Colgan argues that "If the war with Iraq went badly, the regime might have been toppled. Yet the risk to the regime was greatly reduced by Khomeini's use of the state's oil income to fund patronage and consolidate power, even as Iran fought Iraq. Khomeini acknowledged that the oil industry was the lifeblood of the revolution, and could fail without it. Thus, in Iran, oil facilitated the revolutionaries' ambitions, which led to (further) conflict."
Colgan finds that the presence of oil doesn't necessarily make revolution more likely in a given country -- paradoxically, armed rebellions are common in petrostates but successful ones are uncommon. But when there are successful oil-fueled revolutionary governments, those are the states most likely to engage in interstate conflict.
There's an ongoing debate about the relationship between democracy, with a vigorous back-and-forth over questions of causation. But Colgan's book is a good illustration of why the relationship between oil and international conflict isn't as clear-cut as it might seem.
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