America's allies have expressed deep concern about the US intention to deploy a national missile defence. They view it as destabilising to the old certainties of deterrence, epitomised in the 1972 Anti-Ballistic Missile treaty.
Although President Bush appears determined to press ahead, diplomats here note significant changes of emphasis which suggest that a meeting of minds on the issue may not be impossible. The devil, they say, will be in the detail.
In essence, the ABM treaty required the great powers to leave their countries vulnerable to missile attack. Deterrence was based on the understanding that the equal vulnerability of the other side would guarantee their annihilation in a retaliatory attack.
Remove part of the equation and the system collapses.
But, significantly, administration spokesmen have recently omitted talk of "national" missile defence, going some way to respond to fears by allies that a purely US defensive system will leave them even more exposed.
And European leaders have said increasingly that they understand Mr Bush's concerns about "rogue" states and accept the inevitability of the US going down the road of missile defence. Their response has begun instead to concentrate on the shape of such a system.
The last US administration favoured a land-based system which would have intercepted incoming missiles in mid-course or after they re-entered the atmosphere, a technically hugely complicated operation, subject to relatively simple counter measures such as decoy missiles.
Mr Bush, however, has been indicating a willingness to look at sea-based initial-phase interception and space-based systems.
Apart from being technically more viable, a mobile sea-based approach is one that can be deployed close to rogue states and offer protection to others in the region.
Such a system of "theatre" defence is potentially more acceptable to allies, Russia and China alike.
In the dialogue Mr Bush yesterday promised to undertake with them, he is likely to face strong pressure in that direction.
