The U.S. military successfully tested the interception of an intercontinental ballistic missile on Monday, in a first-of-its-kind test of the nation’s missile defense program.

According to U.S. military officials, the intercontinental ballistic missile was fired from the Reagan Test Site on Kwajalein Atoll in the Republic of the Marshall Islands about 4,000 miles from the West coast.

The test was intended to demonstrate how well the U.S. military could respond in the event of a missile being fired from a hostile force such as Iran or North Korea.

“This was the first GBI salvo intercept of a complex, threat-representative ICBM target, and it was a critical milestone,” MDA Director Air Force Lt. Gen. Samuel A. Greaves said in a press release on Monday. “The system worked exactly as it was designed to do, and the results of this test provide evidence of the practical use of the salvo doctrine within missile defense.”

“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,” he continued.

However, Laura Grego, a senior scientist at the Union of Concerned Scientists, told CNBC that the news does mean the missile defense system is ready to defend the U.S. in any such situation, pointing to the secrecy with which the test took place.

“Success is better than failure, but because of the secrecy I have no idea how high the bar was set,” she said. “How realistic was the test? The Pentagon had a very long way to go to demonstrate the system works in a real-world situation.”

The Pentagon has poured billions of dollars into developing its range of missile interceptors, which are based at Fort Greely in Alaska. In next year’s defense budget recently sent to Congress for approval, the Pentagon demanded $9.4 billion for the development of such technology.

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