The U.S. military's THAAD missile-defense system in South Korea is live.
It will take time to become fully operational, but THAAD is now capable of detecting and intercepting short and medium-range missiles. Specifically, North Korean missiles. THAAD was originally deployed last July after a series of North Korean test launches.
THAAD has long been a subject of heated discussion and not particularly well-liked by China. Back in Feburary, China's ambassador to Seoul, Qiu Guohong, said THAAD's deployment would instantly ruin the Seoul-Beijing relationship. China doesn't appreciate North Korea's aggressive weapons tests either. But why? THAAD, at the most basal level, is a protection for South Koreans from North Korean nuclear weapons. THAAD can only intercept missiles during the terminal phase. What it cannot do is destroy missiles mid-course, intercept Chinese missiles, or used to detect really anything regarding Chinese capabilities.
Ultimately, China's dislike for the U.S.-THAAD system is purely strategic in nature. Regardless of where the defense system is stationed, China views it as a threat and yet another U.S. advancement to maintain control of global power. It is no secret that China seeks to grasp some of that power for itself, and it is likely the U.S. missile defense system is seen as a direct threat to China's sovereignty.
Just yesterday, North Korea decided to thank the United States for assuming a harsh stance against its nuclear program. Because of this, North Korea said it will hasten efforts to make progress, which means more test launches. In fact, the statement issued by North Korea touted this effort as "maximum pressure and engagement."
Will be interesting to follow China through this situation and how it handles THAAD if it is needed, North Korean aggressiveness, and President Trump's pretty hard stance on military intervention in the Korean Peninsula.