The U.S. Department of Defense will invest $400 million in MP Materials Corp., acquiring preferred stock that converts into roughly a 15 percent holding and making the Pentagon the miner’s largest shareholder. The agreement forms the core of a multibillion-dollar public-private partnership aimed at creating a fully domestic supply chain for rare-earth magnets, materials critical to everything from electric vehicles to precision-guided weapons. MP Materials will expand production at its Mountain Pass, California, mine and build a second U.S. magnet factory expected to deliver about 10,000 metric tons of magnets annually when it starts up in 2028. The Defense Department has guaranteed a floor price of $110 per kilogram for the two most-used rare-earth elements, while JPMorgan Chase and Goldman Sachs have lined up up to $1 billion in debt financing. The company plans to contribute about $600 million of its own capital. News of the deal sent MP Materials shares soaring nearly 50 percent to their highest level since 2022. Washington’s move comes as manufacturers face tightening supplies after Beijing introduced new export approval requirements. Ford Motor Co. said the paucity of Chinese-made magnets forced it to idle U.S. assembly lines for several weeks and that the situation remains “day-to-day,” underscoring industry vulnerability. The investment coincides with a broader push to bolster domestic supplies. Officials are breaking ground on the Ramaco Brook mine in Wyoming—expected to be the first new U.S. rare-earth mine in 70 years—with an estimated 1.7 million tons of rare-earth oxides. Together, the initiatives mark the most aggressive U.S. effort yet to curb reliance on China, which still accounts for roughly 70 percent of American rare-earth imports.