Both Russia and the US still have the power to destroy the world
By Paul Reynolds
World affairs correspondent, BBC News website
In his nuclear weapons review, President Obama is trying to raise the nuclear threshold and lower the nuclear temperature.
He is taking a step backwards from the policies of President George W Bush, which suggested that a nuclear response might be required if the US faced a significant non-nuclear attack.
Instead, President Obama says, the US will consider using nuclear weapons only in "extreme circumstances" and specifically not against states signing and complying with the Non-Proliferation Treaty.
The review states: "The United States will not use or threaten to use nuclear weapons against non-nuclear weapons states that are party to the Nuclear Non-Proliferation Treaty and in compliance with their nuclear non-proliferation obligations."
This excludes Iran and North Korea. It does not mean that nuclear weapons are an active option against them. It does mean that a theoretical threat is being maintained against the very worst of potential situations.
There is an exception in the case of a significant threat from biological weapons that the US could not otherwise counter.
Nuclear world endures
The president also ruled out developing a new nuclear warhead with the clear statement: "The United States will not develop new nuclear warheads."
And what are called "life extension programmes" for existing warheads will use only components based on previous designs and will not lead to "new military missions or provide for new military capabilities."
The steps announced appear to represent a middle way between the demands of some anti-nuclear activists for more radical measures and the insistence of nuclear weapons supporters that the president should not give away too much.
Mr Obama has sought to balance his narrowing of the conditions under which the US might go nuclear by rejecting a demand that the US declare a "no first use" policy. He has also refused to take US nuclear weapons off a trigger-ready alert, as had been urged by some.
Overall, he seeks both to further the ambitious agenda from a year ago in which he called for a world free of nuclear weapons, and to preserve a chance of getting the Senate to ratify the new nuclear weapons reduction treaty he has agreed with the Russians and the Comprehensive Test Ban Treaty. For that, he needs support from some Republican senators.
Of course, the steps announced do not bring the world that much closer to getting rid of nuclear weapons.
Less terror, more fear
There is a lot of talk about this among governments but limited action. The US retains a mighty nuclear punch; the UK has announced a new generation of nuclear armed submarines; the Russians and the Chinese are busy modernising.
But everyone wants something to show for the Non-Proliferation Treaty review conference in May which will expose nuclear-armed states to further demands from the non-nuclear states for more to be done to fulfil the pledge in the treaty that one day all such weapons will be eliminated.
The new US-Russia Start treaty still leaves each side with the power to destroy the world. But it indicates that they are not interested in blasting each other.
Indeed, much of the thinking behind the review is that a new balance of fear has replaced the old balance of terror. Non-state actors and unpredictable states with nuclear weapon ability or potential are seen as the current or future threat.
Which is why there is a line in the review threatening retribution: "Any individuals responsible for the attack, whether national leaders or military commanders, would be held fully accountable."
As a further measure to support this intensive concentration on nuclear weapons policy, Mr Obama is hosting a conference in Washington on 12-13 April to address the problem of proliferation.