US intelligence suggests that North Korea will have long-range missiles in five years' time, which is making the US military feel deeply vulnerable.
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Rogue states
They argue that the previous threat from the old Soviet Union, may have been hostile but it was predictable.
Now, they say, the threat is from rogue states - such as North Korea, Iran and Iraq - which they describe as unpredictable.
This has created a sense of urgency to get a missile shield in place.
But there are many people who are sceptical that the efforts and the tens of millions of dollars that have been spent on the programme will be worth it.
In January, a prototype of the defence system missed a mock incoming nuclear weapon, 140 miles above the Pacific Ocean.
But, the general in charge of the programme told the BBC that he is confident it will work
"Given the alternative of no defence where all of [the missiles] get through, a system that prevents all but one is better for us," Lieutenent General Ron Kadish said.
Within Nato, however, there is the risk of open division with America protected, the rest not.
Hawkish
British Defence Secretary, Geoff Hoon, told the BBC that it is important there are further talks in Nato among the allies to develop a considered response the US plan.
He says this is under way and has been stimulated by the US.
But some analysts warn of a hostile reaction from Russia
"There is very serious concern that Russia will interpret this new situation as that situation when Russia is vulnerable," military analyst Ivan Safranchur told the BBC.
"This may make Russian policy more hawkish, more aggressive."
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