George W Bush: "Today's world requires a new policy"

That missile defence is needed to protect the US and its allies from rogue states, and that the 1972 treaty banning such systems was an outdated relic of the cold war, which had to go.

"We need a new framework that allows us to build missile defences to counter the different threats of today's world. To do so, we must move beyond the constraints of the 30-year-old ABM treaty," Mr Bush said.

"This treaty does not recognise the present or point us to the future. It enshrines the past. No treaty that prevents us from addressing today's threats, that prohibits us from pursuing promising technology to defend ourselves, our friends and our allies is in our interests or in the interests of world peace."

I am concerned that missile defence... could become a substitute almost for discussion and international diplomacy

Democratic Congressman Neil Abercrombie

But in a clear nod to sceptical allies in Europe and outright opponents such as Russia and China, the president promised to listen to their concerns.

"Today's world requires a new policy: A broad strategy of active non-proliferation, counter-proliferation and defences. We must work together with other like-minded nations to deny weapons of terror from those seeking to acquire them," Mr Bush said.

In the US, missile defence has strong support from most Republicans, but some Democrats, like Congressman Neil Abercrombie from the state of Hawaii, are worried it may replace good old-fashioned diplomacy.

"I am concerned that missile defence... could become a substitute almost for discussion and international diplomacy, the kinds of things that could actually reduce tensions, he said.

Diplomatic hurdles

Technologically, this is all some way off.

The US has not decided on precisely what kind of system to use to try to stop incoming missiles and research is still in its early stages.

The system will face more diplomatic hurdles than technical challenges

Michael O'Hanlon, who has just written a book on missile defence, believes it is feasible, but warns it may not be as effective as some would like.

"On the test range, we will ultimately do very well pretty well. It may take two years. It may take five years, but we will be able to hit a bullet with a bullet or otherwise shoot down a threat," he said.

"The question then becomes what can the potential enemy do by way of a counter-measure to defeat the defence."

But the biggest problem is likely to be diplomatic, not technical, according to Christopher Hellman of the Centre for Defence Information.

"Our European allies remain very concerned about development of the system, particularly if it's going to be one that violates the 1972 Anti-Ballistic Missile treaty, which Europe considers to be the cornerstone of all international arms controls agreements," he said.

"With respect to the Chinese and the Russians, I believe that the United States would deploy a national missile defence regardless of their opposition to the programme," he added.

In a polite way, George W Bush has now effectively said just that, that missile defence is going to happen, preferably with international support, but even without it.