The United States’ Ground-based Midcourse Defense System has had its ups and downs over the years, but finally just showed it has what it takes to defend the country from an inbound nuclear missile launched from a nation like North Korea or Iran.

In a first for the system, a salvo of two ground-based interceptors (GBIs) were launched from Vandenberg Air Force Base in California to intercept an inbound missile fired from the Reagan Test Site on the Kwajalein Atoll in the Marshall Islands more than 4,000 miles away. The first interceptor found its mark, destroying the reentry vehicle that would have housed a nuclear weapon in an offensive launch. The second interceptor arrived immediately thereafter and searched using onboard sensors for a reentry vehicle. When it found the vehicle had been destroyed, the interceptor looked for the next most threatening target (a large bit of debris) and engaged it.

This was the first GBI salvo intercept of a complex, threat-representative ICBM target, and it was a critical milestone,” Missile Defense Agency Director Air Force Lt. Gen. Samuel Greaves said in the statement.

“The system worked exactly as it was designed to do, and the results of this test provide evidence of the practicable use of the salvo doctrine within missile defense,” he said. “The Ground-based Midcourse Defense System is vitally important to the defense of our homeland, and this test demonstrates that we have a capable, credible deterrent against a very real threat.”

Similar tests conducted in 2010, 2013, and 2017 saw failures on at least some level, with one text conducted in 2014 deemed a success. Each of those tests utilized a single GBI to engage the inbound missile, unlike Monday’s test which fired two. In the event of a real offensive launch, multiple interceptors would indeed be fired at the inbound missile, ensuring sufficient redundancy in systems to intercept the weapon even if a number of interceptors fail.

This procedure, however, also demonstrates a significant shortcoming in the Ground-based Midcourse Defense (GMD) System: it’s really only capable of intercepting a handful of inbound nukes before its stores would be depleted. With 44 operational GBIs and plans for a total of 64 in the coming years, the GMD had a standing policy of firing as many as five interceptors at any one inbound nuke. That means the entire arsenal could potentially intercept as many as 12 reentry vehicles, far fewer than a nation like Russia would volley in the event of a nuclear war.

As a result, mutually assured destruction, or the promise America will retaliate in kind if ever made the target of a nuclear strike, still serves as the nation’s most significant form of nuclear deterrent and defense. Systems like the GMD, then, are meant to serve as an additional layer of protection that could intercept rogue launches or missiles fired from underdeveloped nations that could not field as much nuclear firepower. North Korea or Iran, for instance, may be able to get off a single ICBM which the GMD system would be tasked with engaging.

Russia and China are in the process of fielding new intercontinental ballistic missiles with multiple reentry vehicles as well as a number of “dummy” vehicles intended to trick interceptors like those employed by the GMD system.