Colin L. Powell, who in four decades of public life served as the nation’s top soldier, diplomat and national security adviser, and whose speech at the United Nations in 2003 helped pave the way for the United States to go to war in Iraq, died on Monday. He was 84.

He died of complications of Covid-19, his family said in a statement, adding that he had been vaccinated and was treated at Walter Reed National Military Medical Center, in Bethesda, Md., where he died. Mr. Powell had undergone treatment for multiple myeloma, which compromised his immune system, a spokeswoman said.

Mr. Powell was a pathbreaker, serving as the country’s first Black national security adviser, chairman of the Joint Chiefs of Staff and secretary of state. Beginning with his 35 years in the Army, Mr. Powell was emblematic of the ability of minorities to use the military as a ladder of opportunity.

His was a classic American success story. Born in Harlem of Jamaican parents, Mr. Powell grew up in the South Bronx and graduated from City College of New York, joining the Army through ROTC. Starting as a young second lieutenant commissioned in the dawn of a newly desegregated Army, Mr. Powell served two decorated combat tours in Vietnam. He later was national security adviser to President Ronald Reagan at the end of the Cold War, helping negotiate arms treaties and an era of cooperation with the Soviet president, Mikhail Gorbachev.

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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-September 11th 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 that 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 had been “painful” for him personally and would forever be a “blot” on his record.