Rare earth metals go into all kinds of advanced electronics from components of Abrams tanks to wind turbines. While the U.S. once dominated the production of the minerals, 95 percent of the supply now comes from China. In recent weeks, what The New York Times has described as an "embargo on shipments" of the metals out of China has highlighted the insecurity of that supply. Others have suggested a variety of other reasons for the lack of rare earth exports that don't pack the geopolitical punch of an unannounced embargo.
Either way, the fact remains that China controls almost all the supply of a key raw material, and the US is going to have a hell of a time trying to muscle the country's leaders into doing anything they don't want to with them.
One way to solve the problem would be to support domestic rare earth metal production. But that's not going to be easy. While the mine that long produced metals still exists out in California, and there's a company (Molycorp) that wants to get it running, it may not be the cheapest possible way of producing rare earths. But perhaps more crucially, we're missing key expertise because we let the knowledgeable people wander away from the industry when it collapsed. (I'll just note that the same thing has hampered much energy research since the 1970s.)