But until this week, Kofi Annan had couched his criticism in diplomatic terms that stopped short of calling the U.S. action "illegal."
That changed this week. In an interview broadcast by the BBC World Service Interview Program, Annan declared explicitly that the U.S.-led invasion violated the UN charter and hence international law. "I have indicated [the war against Iraq] was not in conformity with the UN Charter," he said. "From our point of view, from the charter point of view, it was illegal."
Analysts say Annan's remarks are likely to reconfirm the feelings of many states that opposed the U.S.-led invasion that they were right to do so. Tim Garden, a security-policy expert at the Royal Institute for International Affairs in London, said that feeling is particularly strong in the French and German governments.
"They have made their decisions and they will feel reinforced that they have got it right. They are both doing significant things for Afghanistan, which they see as the problem that needed to be sorted out before one went doing adventures in Iraq, and I don't think they want to be diverted away from that," Garden said.
Relations over Iraq between Washington and the two European states have been tested again recently over whether NATO should have a role in Iraq and what NATO's relations should be with the U.S-led coalition force there. The alliance agreed in July to send a training mission to Iraq after Washington acceded to French calls not to "prejudge" the relationship with the U.S.-led coalition by blurring the NATO mission with the multinational force.
But Garden told RFE/RL that Annan may have intended his remarks not only to clarify his feelings about events last year but to call for greater multilateralism in possible future crises -- such as Iran. "It may well be that Kofi Annan is making these remarks at this time looking forward to -- with a little bit of concern -- the next international problem, which is not Iraq but Iran, and making it clear that if you want to carry out preemptive interventions, you have got to have the international community behind you and a clear resolution before you go doing it," he said.
The United States accuses Iran of pursuing a nuclear program in order to develop nuclear weapons. Iran says it only seeks nuclear technology for peaceful purposes.
Washington has said it will seek to resolve the crisis peacefully. The UN's International Atomic Energy Agency is currently reviewing Iran's degree of cooperation with arms inspectors.
U.S. and British officials say they had adequate UN authorization for military action against Iraq under Security Council Resolution 1441, which was passed late in 2002. That and previous resolutions called on Saddam Hussein to give up weapons of mass destruction. Security Council Resolution 1441 warned Iraq that there would be "consequences" if it did not comply with UN demands to disarm, but did not specify what those consequences should be.
Annan said in the interview that he believes the U.S.-led coalition preempted UN procedures by going to war without obtaining a second resolution specifically authorizing the forceful disarmament of Iraq.
Britain sought such a resolution early last year but was not able to convince fellow permanent members of the Security Council, France, Russia, and China, that force was needed in place of ongoing UN arms inspections.
Reaction to Annan's remarks has been swift.
Australian Prime Minister John Howard said today that his government believed it acted entirely legally in committing some 2,000 of its military personnel to the U.S.-led action last year. "The legal advice that we had, and I [presented it to the government] at the time, was that the action was entirely valid in international law terms," he said.
But the foreign-affairs spokesman for Australia's opposition Labor Party, Kevin Rudd, called Annan's statement proof that Howard had erred in joining the coalition. "We have always challenged the legality and lawfulness of the Iraq war," Rudd said.
The debate over Annan's remarks come as Australia prepares for parliamentary elections on 9 October. Security and the war on terror are key themes in the campaign debate.
In Britain, the government also rejected Annan's assessment that the invasion of Iraq was illegal. The Foreign Office said that "the Attorney General made the government's position on the legal basis about the use of military force clear at the time."
Britain is due to hold general elections in May or June. British opposition leaders have repeatedly criticized Prime Minister Tony Blair for backing the U.S.-led action against Iraq to remove what he said was a pressing danger posed by Iraq's weapons of mass destruction. Since the invasion, no stockpiles of such weapons have been found.
Washington has not immediately joined the debate over Annan's remarks. The United States is headed toward a presidential election on 2 November. In their election campaigns, President George W. Bush and challenger John Kerry frequently spar over the administration's Iraq policies.
For the latest news on Iraq, see RFE/RL's webpage on "The New Iraq".