The successful interception test over the Pacific this week of a ballistic missile target using the Terminal High Altitude Area Defense system is seen as a message to nuclear-armed North Korea of the technology's capabilities.
Regardless, there are concerns how the controversial anti-missile system might work since it has yet to be battle-tested. Some are worried it could be "overwhelmed" by a swarm-like attack from North Korea, which is known to have hundreds of missiles in its arsenal.
At present, South Korea has two U.S.-supplied THAAD anti-missile launchers deployed. There's also a THAAD battery deployed on Guam, where the U.S. military has bases.
The system is designed to shoot down short and medium-range missiles. Tuesday marked its first intermediate missile intercept test. The intermediate missile has a range of roughly 1,865 to 3,400 miles.
In a press release, the U.S. Missile Defense Agency announced Tuesday it successfully destroyed the ballistic missile after launching an interceptor from a THAAD system located in Kodiak, Alaska. The missile was air-launched by an Air Force C-17 over the Pacific north of Hawaii.
"I'd say it certainly does send a signal that this is a good system for defending against theater-level ballistic missile threats, including North Korea," said Bruce Klingner, former chief of the CIA's Korea branch and now senior research fellow for Northeast Asia at the Heritage Foundation's Asian Studies Center.
Klingner said the reason the U.S. probably didn't test the THAAD system in South Korea was it might have been "seen as counterproductive…and kind of too provocative to the South Korean government."
Unlike the so-called Ground-based Midcourse Defense (GMD) system, which is installed in Alaska and California, the THAAD system is not designed to shoot down intercontinental ballistic missiles. The GMD had its own test May 30, intercepting a mock ICBM fired from California.
"This test further demonstrates the capabilities of the THAAD weapon system and its ability to intercept and destroy ballistic missile threats," Missile Defense Agency Director Lt. Gen. Samuel Greaves said in a statement.
He added, "THAAD continues to protect our citizens, deployed forces and allies from a real and growing threat."
The missile agency said this was the 14th successful intercept in 14 attempts for the THAAD system since 2005.
Critics say, however, that the military's testing may not reflect the danger of swarm-type attacks by multiple incoming ballistic missiles from North Korea or other enemies. Such a scenario could overwhelm or confuse the system and render it useless.
"While the THAAD system does have a good number of interceptors, I can imagine it getting overwhelmed by sheer numbers," said Laura Grego, a missile defense expert and senior scientist in the Global Security Program for the Union of Concerned Scientists.
Similarly, Grego said the U.S. military's May 30 test of the "less mature" GMD system designed to protect the U.S. homeland from ballistic missiles has flaws and was "not challenged in the way they would be [in] a real-world scenario."
"We generally say they are scripted for success. They don't have challenging countermeasures of the type that a really dedicated adversary would include," Grego added.
As for South Korea's vulnerabilities to a swarm-like attack, the North is known to have short-range Scud and other missiles targeting Seoul. There's also the possibility of an attack using submarine-launched ballistic missiles, which Pyongyang is believed to have tested at a naval facility on its west coast.
According to the Missile Defense Agency, "North Korea fields hundreds of Scud or No Dong missiles" that can reach American troops stationed in South Korea and Japan. The agency also pointed out that the North has demonstrated it can fire off a "simultaneous, salvo launch" of medium-range ballistic missiles.
Lockheed Martin, the maker of THAAD, contends its system is designed "to counter mass raids with its high firepower" of up to 72 interceptors per battery, according to its website.
Still, the system hasn't been tested against "complex countermeasures," which could potentially trick the radar, according to experts.
THAAD's interceptor technology also isn't designed to go after missiles in the boost phase, which is an early stage flight. If it could, that would destroy it before it gets time to start countermeasures.
The defense system has never been used in an actual combat situation
There's also the question of range.
THAAD may offer little help for Seoul residents since the capital is considered out of the interceptor's operation range.
The exact range of the THAAD system is believed to be up to 200 km (124 miles). The system was installed in Seongju, about 227 km (or 135 miles) southeast of the capital, Seoul.
Nearly 26 million people live in the Seoul area, meaning there's about half of the country's population that might not be protected with the THAAD system.
The commander of the Eighth U.S. Army in South Korea, Lt. Gen. Thomas Vandal, was quoted as telling Yonhap News Agency on Tuesday that the THAAD system still protects more than 10 million residents in the southern portion of the country. He also indicated the U.S. worked with the South Korean defense ministry when it selected the deployment site, a former golf course.
The U.S.-provided THAAD system costs just over $1 billion for each battery, and President Donald Trump told Reuters in April that Seoul should pay for it. That idea didn't go over well with the South Koreans, some of whom were already feeling economic anxiety from Chinese retaliation over the THAAD.
Beijing contends the system's powerful radar gives the U.S. and South Korea capability not only to spot missiles fired from North Korea, but to potentially look deep into China to monitor military activities. Moscow also has protested the THAAD system and the GMD system installed to protect the U.S. homeland.
South Korean President Moon Jae-in has been receiving political and economic pressure from China to abandon the missile defense system.
Besides the two THAAD launchers currently deployed in South Korea, there are four more additional ones in storage, pending an environmental review by Seoul.
Japan has opted for a different missile shield system, the Aegis Ashore, also from U.S. defense giant Lockheed. That land-based system, which costs just over $600 million, is lower priced than THAAD and is designed to offer protection for a bigger area. Japan also uses a sea-based Aegis system, a technology aboard many U.S. warships.