With the fall of Baghdad the divided UN Security Council is faced with a new set of potential crises over Iraq, including the administration of the Iraqi oil-for-food programme, the lifting of sanctions, and the possible return of weapons inspectors.
In an interview, Mr Adolfo Aguilar Zinser, the Mexican ambassador to the UN who holds the rotating presidency of the Security Council, said one of the most pressing problems was getting a number of new resolutions passed to cope with a new legal situation.
One of the first diplomatic casualties of the war has been the Iraqi ambassador Mohammed al Douri, who "has acknowledged that he does not represent a government that is functioning", said Mr Zinser.
"The government of Iraq has evidently collapsed but has not been replaced by any other authority that could claim the seat of Iraq at the UN.
"The occupying powers cannot represent Iraq at the UN, they are not entitled to that legally. They have certain responsibilities and obligations and de facto they exercise authority to some extent, but they cannot exercise the sovereignty of the state of Iraq vis-a-vis the United Nations."
A number of Security Council resolutions were pending and others "have not been substituted by anything, not even by the realities, because the reason for the involvement of the United States and the United Nations in Iraq was the disarmament of Iraq, and that has to do with a number of legal instruments that were put together by the United Nations, including sanctions and the oil-for-food programme, all are interconnected", Mr Zinser said.
"The first thing that the Security Council has to do is to go back to its original involvement in Iraq and determine what to do in relation to the existence or not of weapons of mass destruction and what the sequence of events would be in terms of sanctions that were imposed in order to disarm Iraq.
"That is something closely tied to the administration of the oil in Iraq, so what decisions the council will take in terms of how this oil is going to be distributed is going to be critical in the very near future."
Under previous council resolutions, disarmament is a condition for the Security Council to lift UN sanctions, including a ban on oil sales for profit, imposed on Iraq after its 1990 invasion of Kuwait.
"Britain and United States will present resolutions when they feel they need legal mandates to transform existing arrangements," Mr Zinser said.
"It's not a matter of the US and the UK necessarily looking for the authority of the Security Council to do whatever they have decided to do, but there are things they cannot do without those resolutions," such as distributing the oil in a different way.
"Oil cannot be exported and the revenues of oil cannot be used outside the programme of oil for food," he said.
The central point of attention in the council will be the reconstruction of Iraq and post-war arrangements and the most important task of the council was to find the path of consensus and understanding among the members, he said.
One of the challenges facing Mr Zinser, formerly an independent-minded left-wing legislator, is overcoming the legacy of the bitter pre-war debates at the Security Council, when Mexico was one of the countries that refused to back a new resolution authorising military intervention.
Mr Zinser said, "I don't want to comment on that" when asked about reports in the Mexican media that Washington had asked Mexican president Mr Vicente Fox to dismiss the ambassador over his role in co-ordinating undecided nations on the Security Council.
The differences over the war still existed, he said. "We never endorsed this war and we never liked this war. We always considered there was another way to disarm Iraq." But he said that "is all in the past" and "we are now moving ahead to find a role for the United Nations" and to find compromises.
UN Secretary-General Mr Kofi Annan said yesterday it appeared there was no functioning government in Iraq, and establishing law and order must be a top concern for the US-led forces in the country.
Under international humanitarian law, the responsibility for maintaining order rested with the invading US, British and Australian forces once they had taken control, he said.
Mr Annan also said he expected UN weapons inspectors to return to Iraq after the war as their mandate from the Security Council to verify Iraqi disarmament remained in force.
Apart from the US, all council members including Britain want the return of UN inspection teams to complete their work.