President Obama’s plans for reducing America’s nuclear arsenal and defeating Iran’s missiles rely heavily on a new generation of antimissile defenses, which last year he called “proven and effective,” but now, a new analysis being published by two antimissile critics, at M.I.T. and Cornell, casts doubt on the reliability of the new weapon.

A high-powered laser aboard a modified Boeing Co 747 jumbo jet shot down an in-flight ballistic missile for the first time, highlighting a new class of ray guns best known from science fiction.

The United States will test its core missile defenses for the first time in January against a simulated long-range Iranian attack, a top Pentagon official said on Monday, amid tensions with Tehran.

A planned U.S. missile shield to protect Europe from a possible Iranian attack would be ineffective against the kinds of missiles Iran is likely to deploy, according to a joint analysis by top U.S. and Russian scientists.

The U.S. military yesterday shot down a missile in a test simulating a long-range ballistic missile attack by a potential adversary such as North Korea or Iran, senior defense officials said.

The Pentagon's missile-defense program has already cost $100 billion and strained relations with the Russians, and it has yet to prove its real-world value. Hitting a bullet with a bullet -- the heart of the system -- is a difficult enough task. But a new report from the U.S. Government Accounting Office reveals that the Pentagon is even having problems launching the practice targets needed to test the system.

U.S. military forces launched a rocket interceptor that destroyed a mock warhead in outer space, giving the Pentagon a much-desired win in its costly and controversial effort to develop a defense against long-range missile strikes.

In the first full-scale test of the ballistic missile defense system in more than a year, an interceptor rocket launched from California on Friday shot down a target fired from Alaska that officials said in some respects resembled a warhead from a North Korean rocket.

Missile defense proponents view North Korea's firing of seven missiles as the first real test of the new U.S. ground-based missile defense, even though the failure of the long-range Taepodong-2 seconds after launch prevented the Pentagon from responding with a U.S.-based interceptor.

After an embarrassing series of test failures in the ambitious, expensive and highly criticized program to build a national missile-defense shield, the U.S. military is slowing the deployment of interceptors while it conducts more testing.