As secretary of state, Colin L. Powell was a reluctant warrior in President George W. Bush’s push to invade Iraq after the Sept. 11 terrorist attacks. He warned the president that it could destabilize the Middle East, upset oil markets and divert political will and resources from the unfinished fight against Al Qaeda.

In a two-hour meeting with Mr. Bush on Aug. 5, 2002, Mr. Powell laid down what became known as the Pottery Barn rules: “You break it, you’re going to own it.”

Mr. Powell did not recommend whether the country should go to war or not — that he believed was the president’s prerogative — but outlined options. After a failed diplomatic effort to avert a conflict, Mr. Bush turned to Mr. Powell to bolster the administration’s case for use of force if the Iraqi leader, Saddam Hussein, did not comply with international demands.

In a 76-minute speech at the United Nations on Feb. 5, 2003, Mr. Powell pressed the American case for a possible war to disarm Iraq, presenting photographs, electronic intercepts of conversations between Iraqi military officers and information from defectors aimed at proving that Saddam Hussein posed an imminent danger to the world.

In the Bush administration’s most explicit effort to connect the activities between Iraq and Al Qaeda, Mr. Powell suggested that Iraq’s lethal weapons could be given at any time to terrorists who could use them against the United States or Europe.

He provided new details about Iraq’s effort to develop mobile laboratories to make germ weapons. He asserted that Iraq had sought to hide missiles in its western desert. Significantly, he cited intelligence reports that Saddam Hussein had authorized his military to use poison gas if the United States invaded.

Before the speech, Mr. Powell had spent several days at the C.I.A. grilling analysts on the intelligence, paring back many of the claims in an early White House draft of the speech that he felt were unsupported. Now he felt confident, he told aides before the address in New York.

“Leaving Saddam Hussein in possession of weapons of mass destruction for a few more months or years is not an option, not in a post-Sept. 11 world,” Mr. Powell declared.

The speech failed to persuade many skeptics in the international community, but Mr. Powell’s personal appeal swung many Americans to support the war, however reluctantly. After American troops invaded in March 2003, however, it became clear there were no weapons of mass destruction. The intelligence had been wrong.

Two years later, Mr. Powell told Barbara Walters of ABC News that his speech to the United Nations was “painful” for him personally and would forever be a “blot” on his record. “I’m the one who presented it on behalf of the United States to the world,” Mr. Powell said, acknowledging that his presentation “will always be a part of my record.”