Speaking today in Vietnam at an Asian regional security meeting, U.S. Secretary of State Hillary Clinton warned the U.S. is prepared to intervene in a dispute involving China’s claim to islands in the South China Sea. The 200 islands have been a source of conflict ever since China took full control of one group of islands, the Paracels, from Vietnam in 1974. This January, China announced that it was going to develop the Paracels as a tourist destination. Clinton’s announcement comes at a particularly tense time between the U.S. and China. On Thursday, she irked Chinese officials by announcing joint U.S.-South Korea naval exercises off the Korean Peninsula. Beijing officials told two U.S. diplomats last March they would not stand for any interference in the South China Sea.