China has developed a monopoly in rare earth metals, a family of 17 metals that are key in the development of wind turbines, lithium batteries, semiconductors, and basically every piece of technical everything.
The US used to have mines, but they were closed due to price wars. Vietnam and Thailand have some undeveloped mines... but a lot of the international sources are owned by China.
China's already restricting supply - they have enough to supply the world with as much as it needs for a long, long time, but they instead provide only 10% of world demand outside of China in a move to try and get other countries to move their R+D and manufacturing to China.
Yet another move China has made right under our nose, with no response from us.
Via http://www.alternative-energy.com.au/ (in the comments section), there are three major companies in Australia who own most of the rare earth metal resources in Australia. The article posted above indicates China moving in on a bunch of the Australian assets already, so this is valuable color. Thanks, Sparty.