Here's What You Need To Remember: Currently, the United States operates THAAD both in the continental United States and abroad. The U.S. Army operates six batteries, including a battery on the island of Guam and a detachment currently being set up in South Korea, to counter North Korea’s growing missile capabilities. The United Arab Emirates also has a THAAD battery, deployed to defend its territory from regional missile threats.

The antiballistic missile that promises to defend South Korea from North Korean nukes has its origins in a Cold War requirement to defend U.S. forces from Soviet tactical missiles. Despite a rocky development history, the Terminal High Altitude Area Air Defense (THAAD) missile defense system now appears to be Seoul’s best defense against a similar attack from the north.

In the late 1980s, the U.S. Army had a requirement for a system to shield ground troops from the threat of ballistic missiles. The Soviet Union integrated ballistic missiles into its force structure down to the division level, with a battalion of SS-21 “Scarab” short-range missile launchers integrated into every motor rifle and tank division stationed in eastern Europe. The SS-21 had a range of seventy-five miles. Further up the chain, the Soviets fielded the SS-23 “Spider,” the replacement for the venerable Scud missile, with a range of 310 miles. Finally, the SS-20 “Saber” intermediate-range missile could be expected to strike tactical targets throughout Europe at ranges of up to 3,200 miles.

The Army’s new medium-range air-defense missile, the Patriot, was not designed to shoot down incoming missile warheads, a very different task from the Patriot’s mission of downing low-altitude strike aircraft. The decision was made to develop a separate system, and in 1987 the system that would become THAAD was born.

The end of the Cold War, and the dissolution of the USSR and Warsaw Pact, at first glance appeared to end the ballistic-missile threat. However, the 1990 invasion of Kuwait exposed a gaping hole in the coalition opposing Iraq’s defenses: Iraqi president Saddam Hussein had hundreds of locally made Al Hussein missiles modeled after the Scud. Patriot PAC-2 missiles pressed into service to defend Israel and Saudi Arabia were originally credited with a high intercept ratio in excess of 90 percent, but subsequent studies revealed few, if any, of the Patriots downed their targets. A tendency for the Al Hussein to break up in reentry, and the Patriot’s delayed proximity fuse, optimized for aircraft, meant many Iraqi warheads made it through the improvised defenses.

The conduct of the war and the continuing proliferation of ballistic missiles worldwide spurred THAAD’s development. North Korea in particular was working to extend the range of its Scud missiles, and during the 1990s was believed by analysts to share data on missile tests with Iran. The CIA also believed that North Korea had sold extended range Scud C missiles (370 miles, versus 190 miles for Scud B) to Iran and Syria.

THAAD is designed to counter mass missile raids, a holdover from the days when large numbers of Soviet missiles were deployed in Europe. A single battery has forty-eight missiles at the ready and can control an additional twenty-four. The AN/TPY-2 radar has a range of 540 miles in intercept mode, and can track up to one hundred targets at once.

THAAD was supposed to be part of a multitiered solution to theater missile defense. With its long range, THAAD could defend a larger volume of airspace, both farther and higher than the Patriot missile. Patriot PAC-3, a BMD-specific variant of the Patriot missile, would defend point targets against warheads that managed to leak through THAAD’s watch.

THAAD first began flight testing in 1995 to test the propulsion, kill-vehicle and guidance systems. Early intercept tests were outright failures, with six misses in a row attributed to hardware malfunctions, with only a single failure attributable to a design flaw. The last two intercepts, however, were successful.

In 2000, THAAD was judged ready for the next phase: engineering and manufacturing development. By 2005, despite the loss of the factory that produced the system’s rocket motors, testing resumed. In fifteen tests spanning from 2005 to 2012, there were eleven successes, with no misses, one cancellation and three aborts. A test in 2011 simulated an attack by two short-range missiles, each of which was intercepted by THAAD with a single interceptor missile. A 2012 test intercepted a medium-range missile, which involved higher intercept altitudes and was cued by a second AN/TPY-2 radar operating forward of the missile battery.

The use of a second radar points to a broader capability: THAAD’s ability to operate within a linked ballistic-missile defense system. Ballistic missiles move fast and can travel over vast geographic distances, so a single intercept might involve multiple “sensors and shooters.” THAAD can operate in “forward basing mode,” sending radar data to other command centers, radar centers and interceptor batteries. In return, THAAD can be cued by other ballistic-missile defense assets, including other AN/TPY-2 radars and the SPY-1D radar on U.S. and allied navy ships.

Currently, the United States operates THAAD both in the continental United States and abroad. The U.S. Army operates six batteries, including a battery on the island of Guam and a detachment currently being set up in South Korea, to counter North Korea’s growing missile capabilities. The United Arab Emirates also has a THAAD battery, deployed to defend its territory from regional missile threats.

The THAAD detachment in South Korea has drawn considerable criticism from China. While China would be expected to be unhappy with an increase of American forces so close to its border, at least part of the unhappiness seems to stem from the AN/TPY-2’s forward basing mode, which dramatically increases the range of the radar from an estimated 540 miles to 1,100–1,200 miles. Forward basing mode would make it possible to “see” deep into China, observing Chinese missile launches and giving U.S. missile defenses a leg up against Chinese attacks. This would weaken China’s nuclear-deterrence strategy.

The United States continues to push the limits of THAAD’s capabilities. In the third quarter of 2017, yet another test will be simulated against an intermediate-range ballistic missile, which will test THAAD’s ability to intercept high-altitude targets. Lockheed Martin has proposed a “THAAD-ER” (Extended Range) to handle the threat of long-range ballistic missiles and new hypersonic weapons which have been researched by China and Russia. According to LockMart, THAAD-ER would incorporate a second-stage rocket motor, giving it nearly three times the range of the current missile and making it capable of defending nine to twelve times the airspace.

THAAD had an extraordinarily long, at times controversial, development period. Going from program inception (1987) to the first unit fielded (2008) took twenty-one years. In many cases, such as the Zumwalt-class destroyers, the nature of the threat may change so much during a lengthy development that the weapon itself becomes a white elephant of questionable utility. In THAAD’s case, the system has tracked pretty consistently with the potential missile threat, while still having the ability to grow to meet future threats. THAAD will likely be defending U.S. and allied forces and population centers for decades to come.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009, he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami. This article first appeared several years ago.

Image: Reuters.