The US missile defence system was put to its first real test on Tuesday and Wednesday with North Korea's launch of a long-range missile and a half dozen shorter range missiles.
Pentagon officials were circumspect, though, about how the multi-billion dollar system performed.
"What I will tell you is that each and every launch was detected and monitored, and that interceptors were operational during the missile launches that took place," said Pentagon spokesman Bryan Whitman.
"The commander of NORAD (the North American Air Defence Command) was able to determine rather quickly that the missiles didn't pose a threat to the United States or its territories," he said.
The North Korean Taepodong-2 was a dud, failing within 40 seconds of lifting off from a launch pad in eastern Korea, so no US interceptor missiles were fired, according to defence officials.
The other six medium-and short-range missiles landed in the Sea of Japan.
It was unclear on Wednesday to what extent the integrated US missile defence system was used to detect and track the launches.
A network of early warning satellites and radars is supposed to feed data to a command centre in Colorado Springs, Colorado, where they are processed by powerful computers linked to interceptor missiles in Alaska and California.
The United States has early-warning radars in Clear, Alaska, and Beale Air Force Base in California, as well as a decades-old network of satellites, that typically would be used to detect missile launches.
But the missile defence system has a broader constellation of sensors to draw on.
Eleven Aegis warships with Spy-1 satellites have been modified for missile defence tracking missions. Two early-warning radars also have been upgraded to track warheads through space.
And new X-band radars have been developed that are supposedly powerful enough to target an object the size of baseball over California from the US East Coast.
The Missile Defence Agency last month deployed the first sea-based X-band radar off Hawaii and a Forward Based X-band Transportable radar to Shariki, Japan.
But a spokesman for the agency said he did not know if they were ready for use in the current situation.
Since the mid-1980s, the United States has spent more than 90 billion dollars to develop a defence against long-range missile attack, a quest that has been fraught with controversy.
The fruit of the effort is what Pentagon officials say is a rudimentary defence against a limited missile attack by North Korea, consisting of nine interceptor missiles at Fort Greely, Alaska, and two more at Vandenberg Air force Base in California.
The interceptor missiles are designed to destroy an incoming warhead in space.
In practice, however, the system has succeeded only in five of 10 attempts to intercept a mock warhead in space. The last intercept occurred in 2002, and that was followed by two failures.
Critics contend that even the successful tests were bogus because they were operationally unrealistic and used surrogates for components of the system that were still in development.
The Government Accountability Office, a congressional watchdog, concluded in a report last year that the system was "uncertain and unverified" because of inadequate testing.
In a May 31 report, it said planning for missile defence operations is incomplete although the military has conducted some training and exercises.
But General Henry "Trey" Obering, head of the US Missile Defence Agency, expressed high confidence in the system last month as the crisis with North Korea was brewing.
"Based on the testing that has been done to date, I am confident that we would hit a long-range missile that was fired at the United States," he said.