China produces the entire world’s supply of samarium, a rare earth metal that the United States and its allies need to rebuild inventories of fighter jets, missiles and other hardware.China’s strict controls on the export of heat-resistant magnets made with rare earth minerals have exposed a major vulnerability in the U.S. military supply chain.Without these magnets, the United States and its allies in Europe will struggle to refill recently depleted inventories of military hardware.For more than a decade, the United States has failed to develop an alternative to China’s supply of a specific kind of rare earth crucial for the...