The Terminal High Altitude Area Defense (THAAD) is an advanced defensive anti-missile system that incorporates a long-range radar used to track incoming ballistic missiles in their terminal phase of flight.
The system consists of multiple missile batteries coordinated by a radar and tracking system. While now initially operationally capable it will not be fully operational for a few months.
The system's deployment to South Korea was agreed to in July of last year, but the initial components of the THAAD system did not arrive in South Korea until the first week in March. The arrival coincided with a North Korea's launch of multiple ballistic missiles.
The system's deployment is receiving a mixed response in South Korea. Early last week protesters greeted the system's components as they arrived at its operational location, a former golf course, located 130 miles southeast of Seoul.
On Wednesday Adm. Harry Harris, the commander of U.S. Pacific Command told Congress last week that the system would be operational "in coming days".
The anti-missile system has also become an issue in South Korea's presidential election being held on May 9.
Moon Jae-in, the leading presidential candidate, has criticized the THAAD deployment and believes that it should be up to a new administration to decide whether the system should be deployed.
The United States has maintained that the system is purely defensive, but China has criticized the system's radar as a potential hedge to its ballistic missile development. China has placed sanctions on major South Korean businesses to show its disapproval of the system's deployment.
"I find it preposterous that China would try to influence South Korea to not get a weapon's system that's completely defensive against the very country that's allied with China," Admiral Harris told a House panel last week. "If China wants to do something constructive then they ought to focus less, in my opinion, on South Korea's defensive preparations focus instead more on North Korea's offensive preparations."
The THAAD missile system will add another layer of anti-missile defense for South Korea. The U.S. military and South Korea both have Patriot anti-missile batteries that can defend against short range missiles.
"But what the president has asked us to do is to look across all of our alliances and to have appropriate burden-sharing, responsibility-sharing," said McMaster. "We are looking at that with a great ally, South Korea. "
"The question of what is the relationship on THAAD, on our defense relationship going forward, will be renegotiated as it's going to be with all of our allies' said McMaster.