Tensions in East Asia are rising after the U.S. began sending parts for a missile defense system to South Korea on Monday.
The Terminal High-Altitude Area Defense system, or THAAD, works by sensing incoming missiles and shooting them out of the sky.
The U.S. currently has other THAAD systems in Hawaii and Guam.
But the THAAD system isn't perfect. A North Korean monitoring group says a nuclear weapon destroyed in the air could still leak material that harms people on the ground.
Also, the system might get overwhelmed if it tries to take out multiple ballistic missiles at once. A day before the U.S. started shipping it to South Korea, North Korea launched four test missiles.
The monitoring group says having two THAAD systems in South Korea could protect the whole country from the North, which the U.S. military says is the only area the defense system will focus on.
But China has long considered that putting the system in South Korea is a threat. One expert says the nation fears THAAD radar could be used to spy on China.
South Korea's government says it believes China could respond to THAAD with a trade war. China is South Korea's largest trading partner.