To China, the US's Terminal High Altitude Area Defense system poses an existential threat.
China has vocally opposed the deployment of the US's Terminal High Altitude Area Defense system for years, despite the system having no warheads, because Beijing may believe it could one day leave it defenseless to a nuclear attack.
THAAD was recently deployed to South Korea for its ability to spot, track, and destroy shorter-range ballistic missiles like the ones North Korea threatens to arm with nuclear warheads and rain down on US forces and South Korean cities.
But despite THAAD being the world's most advanced system of its type, its missile batteries don't scare China. The nonoffensive missiles themselves don't even carry warheads, since they are used solely for crashing into an incoming missile.
The Diplomat speculates that China is scared of the THAAD's powerful AN/TPY-2 radar, which has two modes. The first, for THAAD's announced purpose, can see about 370 miles into North Korea and track and knock down any incoming missile launches.
In another configuration, THAAD becomes a node for a larger ballistic-missile defense system, such as the ground-based midcourse defense that the US recently used to shoot down a mock intercontinental ballistic missile over the Pacific.
Estimates of the range of this forward-basing mode of THAAD vary from a few hundred miles to almost 2,000 miles, meaning any missile launched from mainland China to the US would most likely be spotted very quickly.
THAAD could then transmit that data to other ballistic-missile defense sites and better prepare a counter. China has nothing close to that capability to counter the US.
China doesn't fear THAAD's inert warheads or a South Korea soundly defended against its missile attacks, but an early-warning system that could cripple its nuclear deterrent.
If the US launched a nuclear first strike against China, THAAD would immediately spot the return fire and alert US missile defenses to shoot it down. Although the US's ballistic-missile defense can't yet intercept a high volume of incoming missiles that incorporate countermeasures, that day may come soon.
To China, THAAD poses an existential threat. China has spent billions developing a nuclear arsenal to ensure its sovereignty and ability to deter a great power from attacking it. Having THAAD on its border puts that in jeopardy.