In a significant shift after half a century of nuclear deterrence based on the threat of massive retaliation, the revised doctrine would allow pre-emptive strikes against states or terror groups, and to destroy chemical and biological weapons stockpiles.

Presidential approval would still be required for any nuclear strike, but the updated document, the existence of which was confirmed by the Pentagon at the weekend, emphasises the need for the US to adapt to a world of worsening proliferation of weapons of mass destruction (WMD) in which deterrence might fail. In that event, it states, “the United States must be prepared to use nuclear weapons if necessary”.

The Doctrine for Joint Nuclear Operations, last revised ten years ago, extends President Bush’s doctrine of pre-emptive war to cover a US nuclear arsenal that is expected to shrink to between 1,700 and 2,200 warheads by 2012.

It was drafted by the Pentagon in March and posted on the internet, but did not attract widespread attention until a report on it in The Washington Post yesterday. It has since been removed from the Department of Defence website.

It came to light as Iran insisted, in defiance of the European Union, that it would continue processing uranium at its Isfahan reactor. The US has called on the UN Security Council to impose sanctions on Tehran for failing to shelve its nuclear programme.

Referring repeatedly to “non-state actors” — parlance for terrorists — the doctrine is designed to arm the White House and US forces with a new range of threats and sanctions to counter the situation of threatened nuclear attack by al-Qaeda or one of its affiliates.

The document’s key phrase appears in a list of pre-emptive nuclear strike scenarios, the first of which is against an enemy using “or intending to use WMD”.

Elsewhere it states that “deterrence of potential adversary WMD use requires the potential adversary leadership to believe that the United States has both the ability and will to pre-empt or retaliate promptly with responses that are credible and effective”.

The 1995 version of the doctrine contained no mention of pre-emption or WMD as legitimate nuclear targets.

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