China unilaterality expanded its air-defense identification zone (ADIZ). The new ADIZ includes the Japanese-run Senkaku/Diaoyu islands and the Korean-controlled Ieodo Reef. China's decision triggered United States intervention; the U.S. sent two unarmed B-52 bombers into the contested airspace without informing Beijing's authorities. Korea responded by expanding its own defense zone and conducting a joint military drill with Japan inside the controversial defense zone. Early in December, U.S. Vice President Joe Biden met with authorities in China, Japan and Korea in an effort to avoid an escalation and to tame the ongoing tensions.