By Barnaby Mason
BBC diplomatic correspondent
The question of whether a war on Iraq is legal under international law will become more pressing if the United States and Britain fail to get a new Security Council resolution.
Annan says unilateral action would defy UN charter
They say the use of force would be legal but the UN Secretary General Kofi Annan says unilateral action taken outside the Security Council would not conform to the UN charter.
The UN charter allows military action in self-defence and that can include a pre-emptive strike - but only if there is an imminent threat.
Few accept that Iraq poses such a threat.
So the question comes back to authorisation by the Security Council.
Even the draft resolution now being debated is not clear-cut on this point; if it fails to pass, the key text is last November's resolution 1441.
That gave President Saddam Hussein a final opportunity to give up weapons of mass destruction.
But as for what would happen if he did not do so, 1441 merely reminded him of the serious consequences threatened in previous resolutions.
It did not authorise member states to use all necessary means to enforce it - the key phrase in Security Council resolution 678 that gave authority for the expulsion of Iraqi forces from Kuwait in 1991.
The reason 1441 was ambiguous is that France and Russia would not agree to such clarity.
Many experts in international law say the absence of explicit wording means that 1441 does not authorise force.
1441 gives Saddam a final chance to disarm
But some agree with the UK Government that it may rely on previous resolutions that did.
To sum up the argument: the Security Council authorised war to end the occupation of Kuwait; the ceasefire in 1991 was conditional on Iraqi disarmament; so Iraq's failure to disarm revives the authorisation to use force.
This is controversial stuff - and certainly no Security Council resolution authorises toppling Saddam Hussein.