President Donald Trump signed an executive order on Tuesday that launched a national security investigation into the U.S. reliance on imports of critical minerals, after China suspended exports of certain rare earth metals and magnets.

Trump on April 2 stunned the world with massive tariffs that he said would prevent countries like China from “ripping off” the U.S. Chinese leader Xi Jinping ditched official caution and issued a patriotic message casting doubt on whether American voters could bear as much hardship as the Chinese.