At the Union of Concerned Scientists blog All Things Nuclear, David Wright writes that:
"the Obama administration's approach to missile defense has been particularly disappointing -- and is potentially dangerous. Originally the administration said it would require missile defenses to be 'proven,' . . . So it was surprising when (a) the administration’s Ballistic Missile Defense . . . Review stated that 'The United States is currently protected against limited ICBM [intercontinental ballistic missile] attacks,' and (b) the President called the Aegis missile defense system 'proven' in the announcement of his proposed European system in September 2009."
"Neither of these statements are [sic] true in any meaningful sense. Neither the Aegis system nor the Ground-Based Midcourse Defense (GMD) system fielded in Alaska and California has been subjected to realistic tests against the kind of attacks and under the conditions you would expect in the real-world."
Wright goes into more detail.
The Pentagon is using sleight of hand: it is defining the "threat" very narrowly. [It] has defined a "limited missile attack" as an attack by a limited number of missiles, and by missiles that have no countermeasures. . . . But it makes no sense to assume that North Korea, Iran, or any other country would spend years developing a long-range missile to hit the U.S. . . . and not have some of its aerospace engineers also design countermeasures that would make the missiles effective against [U.S. missile defense. After all] effective decoys and other countermeasures can be built with less sophisticated technology than is needed for a long-range missile and nuclear warhead. [Emphasis added.]
Then Wright demonstrates the threat that hyping missile defense can pose to national security.
First, if military and political leaders believe they have defensive capabilities that they do not in fact have, that can lead them to make bad decisions. For example, if [they mistakenly believe that] they have effective anti-missile systems it may encourage them to take aggressive actions that are in fact likely to make another country launch missiles at them.
[Second] the claim that Aegis is "proven" has led officials to believe the U.S. should buy and deploy many hundreds of Aegis interceptors before they have actually been shown to be effective.
Wright sums up:
It would be ironic if the administration's real steps to reduce nuclear threats to the United States were derailed . . . by its pursuit of a system with known shortcomings that has yet to undergo realistic testing.