In a February 24 article on Senate Minority Leader Mitch McConnell's (R-KY) efforts to block a Democratic proposal to repeal the 2002 Iraq war authorization, The Washington Post uncritically reported that White House deputy press secretary Tony Fratto “said the United States went [into Iraq in March 2003] as a multinational force under United Nations authorization to take military action against Iraq.” The Post failed to note that Fratto's claim is disputed. On March 19, 2003, just before the invasion of Iraq, the Post itself reported (purchase required) that President Bush tried and failed to obtain a resolution from the U.N. Security Council clearly authorizing new military action against Iraq:

From Beijing to Mexico City, governments expressed regret that the United States had given up on diplomatic efforts at the United Nations to disarm Iraqi President Saddam Hussein's government. Many said war was not justified at this time, especially without the blessing of a new United Nations Security Council resolution that Washington unsuccessfully sought.

The weblog Think Progress also debunked Fratto's statement, pointing to a Post article from March 18, 2003, reporting that the Bush administration failed “in its months-long effort to win the blessing of the U.N. Security Council to forcibly disarm the Iraqi leader”:

Earlier in the day, British and U.S. diplomats, facing certain defeat on the Security Council, withdrew a resolution that would have cleared the way for war. Though Bush on Sunday vowed another day of “working the phones,” it quickly became clear that as many as 11 of 15 council members remained opposed and the effort was abandoned by 10 a.m.

The withdrawal of the resolution without a vote was a double climb-down for Bush. On Feb. 22, he had predicted victory at the United Nations, and on March 6 he said he wanted a vote regardless of the outcome.

The Bush administration maintains that the 2003 invasion of Iraq was justified by Saddam's violations of several earlier U.N. Security Council resolutions, going back to Iraq's invasion of Kuwait in 1990.

From the February 24 Washington Post article:

Democratic architects of the new initiative, including Reid, Senate Foreign Relations Chairman Joseph R. Biden Jr. (Del.) and Armed Services Chairman Carl M. Levin (Mich.), have argued that the 2002 authorization is no longer valid, because the intent was to destroy weapons of mass destruction -- which were never found -- and if necessary to depose Saddam Hussein, who has since been captured, tried and executed.

The second part of the authorization, Fratto said, “is still important and envisioned the changing nature there.” Even Bush has acknowledged the shift, he said: “The president said this isn't the fight we entered in Iraq but it's the fight we're in.”

Fratto said the United States went in as a multinational force under United Nations authorization to take military action against Iraq and was there as an occupying force. “And now we're there at the invitation of the sovereign elected government of Iraq,” he said, adding that “U.N. Security Council resolutions that came subsequent to the war authorization, you know, envisioned those kinds of changes.”

Biden responded in a CNN interview that while the Constitution allows Bush to conduct war, it does so “only if the Congress gave him the authority in the first place.”