UN Security Council Resolutions 678, 687 and 1441. The administration also cites Article 51 in the UN Charter, the inherent right to self-defense; and Chapter VII, the right to use force to restore or maintain international security.

USG texts providing an explanation of the legal position:

In an interview on 26 March 2003, Secretary Powell states:
This war is being conducted under the authority of UN Resolution 1441 and earlier Resolutions 678 and 687. 1441 is the most relevant one because last November by a vote of 15-0 the Security Council said this was Iraq's last chance.

http://usinfo.state.gov/regional/nea/iraq/text2003/0326powell.htm

In Amb. Negroponte's Letter to UN Security Council President Traore 20 March 2003, he writes: The actions being taken are authorized under existing Council resolutions, including resolution 678 (1990) and resolution 687 (1991).

These operations are necessary in view of Iraq's continued material breaches of its disarmament obligations under relevant Security Council resolutions, including 1441 (2002).

http://usinfo.state.gov/topical/rights/law/03032109.htm

On 20 March 2003, Dept. of State Legal Advisor Taft also cited the UN resolutions:

Under international law, the basis for use of force is equally strong. There is clear authorization from the Security Council to use force to disarm Iraq. The President referred to this authority in his speech to the American people on Monday night. The source of this authority is UNSCR 678, which was the authorization to use force for the Gulf War in January 1991. In April of that year, the Council imposed a series of conditions on Iraq, including most importantly extensive disarmament obligations, as a condition of the ceasefire declared under UNSCR 687. Iraq has "materially breached" these disarmament obligations, and force may again be used under UNSCR 678 to compel Iraqi compliance.

Just last November, in resolution 1441, the Council unanimously decided that Iraq has been and remains in material breach of its obligation.

http://usinfo.state.gov/regional/nea/iraq/text2003/032129taft.htm

Factsheet on Iraqi Violation of U.N. Resolutions
State Department Details Saddam's Defiance of UN Resolutions, March 20, 2003.
Describes the details of Iraq's disregard for 16 UN Resolutions.

http://usinfo.state.gov/regional/nea/iraq/text2003/0320defy.htm

U.S. Authorization for the Use of Military Force Against Iraq (Resolution of 2002), P.L. 107-243, Congressional authorization; signed by President Bush on 16 October 2002, reads:

Congress has authorized the President

"to use United States Armed Forces pursuant to United Nations Security Council Resolution 678 (1990) in order to achieve implementation of Security Council Resolution 660, 661, 662, 664, 665, 666, 667, 669, 670, 674, and 677"

Whereas in December 1991, Congress expressed its sense that it

"supports the use of all necessary means to achieve the goals of United Nations Security Council Resolution 687 as being consistent with the Authorization of Use of Military Force Against Iraq Resolution"

http://frwebgate.access.gpo.gov/cgi-bin/getdoc.cgi?dbname=107_cong_public_laws&docid;=f:publ243.107.pdf

This law required a Presidential Determination report before the start of the military action. The Presidential Determination report sent to Congress on 19 March 2003 states: U.S. action is consistent with the UN Charter. The UNSC, acting under Chapter VII of the UN Charter, provided that member states, including the United States, have the right to use force in Iraq to maintain or restore international peace and security. The Council authorized the use of force in UNSCR 678 with respect to Iraq in 1990. This resolution - on which the United States has relied continuously and with the full knowledge of the UNSC to use force in 1993, 1996, and 1998 and to enforce the no-fly zones - remains in effect today. In UNSCR 1441, the UNSC unanimously decided again that Iraq has been and remains in material breach of its obligations under relevant resolutions and would face serious consequences if it failed immediately to disarm. And, of course, based on existing facts, including the nature and type of the threat posed by Iraq, the United States may always proceed in the exercise of its inherent right of self defense, recognized in Article 51 of the UN Charter.

http://usinfo.state.gov/regional/nea/iraq/text2003/0319whitehouse.htm

In a speech 17 March 2003, President Bush made reference to the basis in UN resolutions for the upcoming action: One reason the U.N. was founded after the second world war was to confront aggressive dictators, actively and early, before they can attack the innocent and destroy the peace.

In the case of Iraq, the Security Council did act, in the early 1990s. Under Resolutions 678 and 687 -- both still in effect -- the United States and our allies are authorized to use force in ridding Iraq of weapons of mass destruction. This is not a question of authority, it is a question of will.

Last September, I went to the U.N. General Assembly and urged the nations of the world to unite and bring an end to this danger. On November 8th, the Security Council unanimously passed Resolution 1441, finding Iraq in material breach of its obligations, and vowing serious consequences if Iraq did not fully and immediately disarm.

Today, no nation can possibly claim that Iraq has disarmed.
http://usinfo.state.gov/regional/nea/iraq/text2003/0317bush.htm