There is one missile system straining relations between the United States, China, and South Korea. It is causing cyberattacks, forcing supermarkets to close, and leading to tourism boycotts. It is the THAAD ballistic missile system. Hot on the heels of a North Korean missile test, THAAD is being deployed to South Korea—and neighboring China is hopping mad.

The United States invented THAAD, or Terminal High Altitude Area Defense , in the 1980s as a means of shooting down Soviet tactical missiles. In any war with the USSR, Soviet ground forces would almost certainly use tactical missiles such as the SS-21 Scarab and the Scud missile to strike U.S. and NATO targets, possibly with nuclear, chemical, or biological warheads.

THAAD development began in 1987. The system was not ready for the 1991 Persian Gulf War, when the Iraqi military launched between 85 and 93 Al Hussein missiles , a locally produced version of the Scud, at targets in Saudi Arabia and Israel. The missiles were engaged by PAC-2, the latest version of the Patriot missile with less than optimal ballistic missile defense capability.

North Korean Musudan intermediate-range ballistic missile during launch, June 2016. Via KCNA.

The poor intercept rate of the Patriot missile, the devastating cost of a missile impact, and the threat of ballistic missile proliferation worldwide (especially from countries such as North Korea) bolstered the argument for THAAD into the 1990s. The system was so important that it survived test failures that would have doomed other military tech, including six failed tests in a row out of eleven conducted between 1995 and 1999.

The THAAD system consists of three key subsystems: the THAAD missiles, a mobile tactical operations center, and the AN/TPY-2 radar. The missiles are carried eight per launch vehicle, and THAAD battery has six launch vehicles for a total of 48 missiles. The AN/TPY-2 radar is an X band active electronically scanned array radar similar in design to the radar on an F-22 or F-35 fighter. The tactical operations center (TOC) functions as the headquarters of the THAAD unit. Both the radar and the TOC are trailer-mounted, making the entire system air-transportable and capable of self-deployment on the ground.

Unlike Patriot, which was designed to shoot down aircraft and modified to down ballistic missiles, THAAD was built from the ground up to target missiles. THAAD missiles are thought to have a range of 125 miles, and can intercept targets up to 93 miles above the Earth. While Patriot originally had a 200 pound blast fragmentation warhead to knock aircraft out of the sky, THAAD uses a "kinetic kill" strategy that does away with explosives altogether to directly strike the enemy warhead. This ensures that the enemy warhead is actually destroyed and not merely knocked off course, and that an incoming nuclear warhead doesn't accidentally go off.

THAAD missile test, 2013. Missile Defense Agency

The United States has wanted to deploy THAAD in South Korea for years, if only to protect U.S. troops stationed there from North Korean missile attack. South Korea, afraid of angering its much larger neighbor China, waffled for years. All of that changed in January 2016, when North Korea conducted its fourth nuclear test . Within hours, the U.S. and South Korea had announced they were holding joint talks on THAAD deployment to the Korean peninsula.

The nuclear test was a wake-up call. Previously, South Korea had only limited ballistic missile defenses and was waiting on a THAAD-like, locally-produced system known as KAMD . Although KAMD was taking time to develop it could be sold abroad, an important consideration for export-driven South Korea. Alarmed by Pyongyang's nuclear test, the U.S. and South Korea agreed to deploy THAAD to Korea, and settled on a golf course owned by Lotte, one of Seoul's largest corporations.

China, of course, is not happy. It calls placing THAAD in South Korea a provocative act. There are a number of theories why this is the case. The first is that China opposes any expansion of U.S. power in its sphere of influence as a matter of principle. If Chinese political and military power is growing, the deployment of advanced U.S. missiles near its border would seem to contradict that. Another possible reason is that the AN/TPY-2 radar will be able to see into China, possibly giving the U.S. early warning of a Chinese missile strike and undermining Chinese nuclear capability. In a similar vein the AN/TPY-2 will allow the U.S. to see into China on a regular basis, a legal form of spying.

THAAD missile vehicle rolling off an Air Force transport at Osan Air Base, March 7, 2017. Getty Images

Plans to deploy THAAD were apparently accelerated earlier this month when North Korea fired four Hwasong-7 missiles at once. Within hours, THAAD missiles, minus the vital AN/TPY-2 radar, were rolling off an Air Force C-17 transport in South Korea . The radar and tactical operations center are set to arrive by the end of March .

China is striking back against the deployment with a number of political and economic measures, many of which appear grassroots but are likely government-directed. Fifty-five of 99 Lotte discount stores operating in Beijing (the same South Korean corporation that owned the golf course) were shut down after surprise safety inspections. A Hershey factory co-owned with Lotte (which also makes a lot of candy) operating in Shanghai was also shut down. South Korean K-pop stars have had their gigs canceled in China, and Chinese are organizing tourism boycotts of South Korea.

One of the markets owned by South Korea's Lotte Group shut down for violating fire safety rules on March 13, 2017 in Beijing, China. VCG Getty Images

The irony is that China is cracking down harder on South Korea, which merely wants to protect itself, than it is on a nuclear-armed North Korea. THAAD is an entirely defensive system, early warning and spying concerns aside. If China really wanted to stop THAAD it could have exerted even more pressure on North Korea to stop its nuclear program, although it's disputed exactly how much influence Beijing has over Pyongyang.

Meanwhile, South Korea's government is in turmoil after the impeachment of pro-THAAD President Park Geun-hye . A new left-wing government might cancel deployment of the missile system. A new government might be more willing to negotiate with North Korea, but canceling the system without extracting concessions from the North on nuclear weapons would simply make South Korea, and U.S. troops based there, less safe.

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