President Trump’s decision to launch Cruise missiles against Syria was made without informing other world leaders, including his guest at Mar-a-Lago, Chinese President Xi Jinping. Does Trump know he wouldn’t have those missiles at his disposal without China?

He should.

The guidance systems on the Tomahawk Cruise missiles used to destroy the air base at Al-Shayrat, rely on rare earth elements (REEs) that are made almost exclusively in China.

Those substances are used to make permanent magnets, which oscillate in the tail fins of the Tomahawks, guiding them accurately to their target. They are also used in fighter jets, precision guidance, radar and antimissile defense systems. Highly developed magnets have proven more reliable than electronic guidance systems.

But just like commercial consumers, the Pentagon is wholly reliant on China for imports of the 17 metals and minerals that are collectively called rare earths because they are usually found together. Having demonstrated his willingness to use military force, Trump must now think about how to ensure that force is always available.

“This is a reminder to the Pentagon that rare earths are used in expendable weaponry,’ says Jeff Green, a defense industry consultant. “And those munitions are used in a shooting war, when supplies are most likely to be cut off.”

It is unlikely that President Xi will suspend deliveries to the United States because of Trump’s action Thursday night in Syria. But the U.S. administration is also talking tough about North Korea, which recently tested its own version of a Cruise missile and other weapons, in contravention of U.N. sanctions. China is North Korea’s sole patron and keeps it afloat with economic aid.

Should Trump signal to Xi that he is considering military action against North Korea at their Florida summit, China might decide to flex its own muscle. Cutting off America’s supply of REEs might be one way of doing that.

The Pentagon said that some 60 Cruise missiles were used in Thursday’s attack. It’s estimated that the U.S. has a stockpile of 3,000 left. That’s a large, but not an infinite amount. And REEs are also crucial in the production of AIM Sidewinder and Phoenix missiles used by U.S. warplanes, as well as smart bombs like the Joint Direct Attack Munitions (JDAM).

The United States used to produce its own rare earths, which are found in abundance on American soil. But once China began competing, without regard to the environmental hazards involved, it undercut American companies and drove them out of business.

Relying on China for materials vital to our national security is short-sighted. And attacking Syria with Cruise missiles, while hosting Xi, whose country controls the world’s supply of rare earths, is a vulnerability that should not be lost on a president who promised to make America great again.