As a result, the United States needs radar installations and anti-missile launch sites in key regions, particularly in Central Europe to counter the growing missile threat from Iran. And Russia, the U.S. partner on strategic arms reduction and in the effort to convince Iran not to develop nuclear weapons, has not been at all happy about that.

Russia says the European missile defense system changes the balance of power and threatens its nuclear arsenal. But Andrew Kuchins, director of the Russia and Eurasia program at the Center for Strategic and International Studies, says that's not the real reason for Russian opposition.

"They understand that these systems don't have the capability to compromise the Russian strategic deterrent. What they want to do, I think, is try to block the development of missile defense capabilities in general so as to make it more costly for the United States to deploy conventional forces into the [European] theater," he said.

"Conventional superiority"

But Deputy Defense Secretary William Lynn says the ability to project conventional power without the threat of missile attacks is crucial to the U.S. global security strategy, and its treaty obligations.

"This risk could push our forces further from the battle space, compromising our ability to bring our conventional superiority to bear. The credibility of our security guarantees to allies and to partners especially in the Middle East and East Asia depends on our ability to project power despite these threats," he said.