In a speech delivered shortly before George Bush addressed the UN general assembly on Tuesday, Mr Annan declared that the Iraq crisis brought the UN to a "fork in the road" as decisive as 1945 when the world body was formally established.
The 191 UN members, meeting in New York this week, are struggling to heal deep rifts caused by the war on Iraq, in which the US acted without the approval of the security council.
In a text of his speech released in advance Mr Annan questioned US arguments that nations have the "right and obligation to use force pre-emptively" against unconventional weapons systems, even while they are still being developed.
"My concern is that, if it were to be adopted, it could set precedents that resulted in a proliferation of the unilateral and lawless use of force, with or without credible justification," he said.
He acknowledged that the UN charter allowed military action for the purpose of self defence. "But until now it has been understood that when states go beyond that and decide to use force to deal with broader threats to international peace and security, they need the unique legitimacy provided by the United Nations.
"Now some say this understanding is no longer tenable since an 'armed attack' with weapons of mass destruction could be launched at any time. This logic represents a fundamental challenge to the principles on which, however imperfectly, world peace and stability have rested for the last 58 years."
President Bush was expected to make a defiant stand at the UN, demanding international support in cash and troops for the US occupa tion of Iraq, while rejecting a speedy transfer of authority to Iraqis.
Excerpts of his speech to the general assembly released on Monday made it clear that there remains a gulf between the US and its European critics, led by France, about the governance of Iraq. But both sides are clearly anxious to avoid the bitterness of earlier UN debates on Iraq, offering goodwill concessions.
France promised not to brandish its security council veto, while Mr Bush said the UN could help draw up a constitution for Iraq and play a role in supervising eventual elections.
The timetable for those elections and the transition to Iraqi self-rule is at the heart of the current debate, in which the US is insisting on a delib erate pace, going deep into next year, and the French are calling for a much faster handover.
Condoleezza Rice, Mr Bush's national security adviser, said on Monday that the transfer must follow "an orderly process". She told reporters: "The French plan, which would somehow transfer sovereignty to an unelected group of people, just isn't workable."
According to one version of the text, Mr Bush was due to offer a compromise solution, asking the Iraqi governing council to come up with a schedule for elections.
Most of the council's 25 members are in favour of a fast-paced transfer, and several were in New York this week to lobby for that option. But a diplomat in New York said it was unlikely that the US would surrender the decision to the council without a prior agreement that it would respect Washington's wishes.
Mr Bush was expected to urge UN members to bury their differences on Iraq, send money and soldiers, and focus on other global problems, such as nuclear proliferation in "rogue states", Aids and human trafficking.
President Jacques Chirac, who was scheduled to address the general assembly after Mr Bush, indicated that France would not veto a US-backed security council resolution calling for military and financial support for Iraqi reconstruction, but also made clear that Paris would not answer that call unless authority was handed over to Iraqis promptly.
Meanwhile Tony Blair, who met Mr Chirac and the German chancellor, Gerhard Schröder, in Berlin last weekend in an unsuccessful attempt to bridge the US-Europe divide on UN involvement, has decisively lost the domestic debate over Iraq. A clear majority of British voters now say that the war was unjustified, according to this month's Guardian/ICM poll. In April support for the war peaked at 63%. Now 53% of respondents say the war was unjustified, and only 38% believe it was right to invade Iraq.