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 decision to restrict exports of a range of rare earths and magnets has tripped up the supply chains central to automakers and aerospace manufacturers.

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…