The United States and South Korean air forces are holding one of their largest ever joint exercises. The five days of exercises are named Vigilant ACE. South Korean troops are training with more than 230 airplanes and 12,000 members of the U.S. armed forces. The planes include a number of U.S. stealth warplanes. The high-technology aircraft are designed to avoid discovery by radar. The exercises began less than one week after North Korea tested its most advanced long-distance missile. The North Korean government said it launched an intercontinental ballistic missile, or ICBM, last Tuesday. The Hwasong-15 rocket traveled farther than any of the country’s earlier rockets. The new rocket was said to be able to reach anywhere in the United States.