One issue: It doesn’t really matter where Middle Eastern oil goes—if supply is disrupted, prices go up for everybody. “The origin of whatever molecules are consumed in the United States does not matter,” Kenneth Vincent, an economist at the Department of Energy, said at a 2017 Georgetown conference. “What matters is that if there’s a shooting war somewhere in the Middle East, those molecules will cost more and that will harm the American economy.”

Read: The Iran crisis is forcing Trump into uncomfortable territory

The U.S. is also providing a global public good as the dominant military force in the region. “We’re doing it not just for us; we’re really doing it for the world, and we’re doing it because it impacts us and we’re the only ones really big enough to do it,” says Rosemary Kelanic, a Notre Dame professor who researches energy security. “Other countries have been totally happy to free ride on that.”

This kind of dynamic has drawn Trump’s ire in other areas, such as in Europe, where he has questioned the collective security guarantee at the core of NATO. So the question he raises about the U.S. commitment for Gulf oil supplies is not unusual. Caitlin Talmadge, a security-studies professor at Georgetown University, told me that the U.S. decision to reduce its presence in the Gulf started with Barack Obama’s pivot to Asia and the 2018 “National Defense Strategy,” which emphasized great-power competition with Russia and China. “Along the lines that Trump is talking about, we’ve actually tried for years and years … to get both Gulf states and our allies who depend on Gulf oil to do more to provide collective naval security in the Gulf. And we have made some strides there,” Talmadge said. She cited joint naval patrols, exercises, and training coordinated by the U.S. Fifth Fleet, with regional and allied countries.

Even so, the United States is the dominant military in the region. “The United States has always sort of been the glue that’s holding that coalition of actors together,” Talmadge said.

Read: The world is getting sucked into U.S.-Iran tensions

With its mines and missiles, Iran could block part or all of the strait and then shoot at whatever ships try to clear it. Doing so would damage Iran itself—or mean “cutting its own throat,” as the analyst Michael Knights put it to us recently—given its economy’s dependence on oil exports. But as the Trump administration has moved to cut off Iran’s oil exports entirely, Tehran has lost much of this deterrent to closing the strait or at least harassing shipping nearby.

There remains, of course, the deterrent of the possible U.S. response. “If they close the Strait of Hormuz, they’re going to get bombed,” Kelanic says. “If we didn’t intervene and bomb them and then clear the strait, which we would have plenty of time to do”—given that the U.S. and allied strategic petroleum reserves could make up any oil shortfall for months—“oil supplies would be constricted and prices would go up.”