KUHN: The residents and activists aren't the only ones who want to see the THAAD batteries removed. China fears that the system's radar could be used to spy on it and defeat its missiles. After THAAD was installed, Beijing used undeclared economic sanctions to punish South Korea until Seoul made several promises. One was that it would not install any more THAAD batteries. Another was that the existing batteries in South Korea would not be integrated into the larger U.S. missile defense system. But that's exactly what the U.S. has in mind.

CLINT WORK: The upgrades are really focused on the more immediate threat, which is North Korea. And I think the upgrades will involve, first and foremost, integrating the systems with other existing U.S. assets - obviously, the PAC-3s.

KUHN: Clint Work is a fellow with the Stimson Center in Washington. This year, the U.S. began testing to link THAAD to its Patriot, or PAC-3, missile systems. He says that the idea is to give the U.S. and its bases multiple layers of defenses.

WORK: North Korea's own missile advancements clearly are intended to exploit gaps in the current coverage.

KUHN: In order to do that, the North has recently tested hypersonic missiles and missiles launched from submarines and trains. Work says that THAAD batteries here could also be integrated with other THAAD batteries in Japan and Guam and a central missile defense control center in Alaska. In future, he says, it's possible that THAAD could be repurposed to counter a threat from China.