WASHINGTON — Much like that day 10 years ago, it was sunny and mild Sunday at the Pentagon. But that was the only similarity.

Instead of a massive plume of grayish-black smoke belching from the western side of the Pentagon and buses screeching across the Potomac River from Walter Reed Army Medical Center, there was the somber sound of a bugler playing taps and a military choir softly singing the hymn “Amazing Grace.’’

Rather than hundreds of people streaming out of the crippled building, there was a carefully choreographed ceremony in which members of the four military services laid small wreaths of white flowers on the 184 benches that commemorate those killed when terrorists crashed American Airlines Flight 77 into the Pentagon.

And instead of an Air Force officer shouting that “we need doctors, nurses and EMTs,’’ there was Vice President Joe Biden telling relatives of those killed that Osama bin Laden and the al-Qaida terrorists who attacked New York and the Pentagon “never imagined the sleeping giant they were about to awake.’’

“They did not know us,’’ Biden said of al-Qaida. Rather than terrifying Americans, he said the terrorists “galvanized an entire new generation of patriots – the 9/11 generation. Many of them were just kids on that bright September morning.’’

“Al-Qaida, bin Laden never imagined that the 3,000 people who lost their lives that day would inspire 3 million to put on the uniform and harden the resolve of 300 million Americans,’’ Biden said.

Nearly 1,000 people gathered near the spot where the jet smashed into the Pentagon, producing a massive fireball and a powerful explosion that shook people throughout the building. Fifty-nine passengers and crew on the plane and 125 people in the Pentagon died. One of the passengers was just 3 years old.

Among those attending were House Speaker John Boehner, R-West Chester Twp., former Secretary of Defense Donald Rumsfeld, who that day helped carry wounded from the smoking wreckage, and retired Gen. Richard Myers, former chairman of the joint chiefs of staff.

Defense Secretary Leon E. Panetta, who introduced Biden at the ceremony, told the crowd that “no words can ease the pain you still feel.” He said that the country would never forget the human cost paid by this generation — including “the more than 6,200 soldiers, sailors, airmen and Marines lost in the line of duty” since 9/11.

Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, said, “Today, we stand on this hallowed ground to honor those who still live on in our hearts.”

The grisly attacks on the Pentagon and the twin towers of the World Trade Center prompted President George W. Bush one month later to attack al-Qaida’s safe havens in Afghanistan, toppling their Taliban allies and replacing them with a fragile democratic regime.

Bush also launched a second war in 2003, sending U.S. combat forces into Iraq to end the regime of Saddam Hussein and destroy his expected arsenal of weapons of mass destruction, weapons that Bush and Rumsfeld feared could fall into al-Qaida’s hands.

But when U.S. forces found no evidence of nuclear or chemical weapons, the unity in the aftermath of Sept. 11 created deep divisions in the country. But divisions on Iraq were hidden Sunday as speakers focused instead on America’s successful pursuit of al-Qaida’s leaders – from bin Laden, who was killed this year in Pakistan by U.S. special forces, to Khalid Shaikh Mohammed, the architect of the September 11 attacks, captured in Pakistan in 2003.

Biden said American forces “were prepared to follow bin Laden to Hell’s Gate if necessary. And they got him. My God, do we owe those special ops folks and intelligence guys who got him, many whom who have subsequently lost their lives.’’

“But we will not stop; you will not stop until al-Qaida is not only disrupted, but completely dismantled and ultimately destroyed.’’

In a gesture of conciliation, Biden singled out Rumsfeld, saying he “did what he did as a young soldier and young man and did all his life. You, he and others streamed into the breach between the fourth and fifth corridors where the devastation was the greatest and where death came in an instant, but also where there were some survivors to be found.’’

Both Biden and Panetta hammered home a basic theme: The terrorists, much like the planners of the Dec. 7, 1941, attack on Pearl Harbor, had no understanding of the American character.

“We were challenged by al-Qaida and its vicious hated aimed at squarely at our values,’’ Panetta said. “They tried to weaken us. Instead, they made us stronger.’’

Biden said the terrorists who attacked the Pentagon “sought to weaken America by shattering this defining symbol of our military might and prowess. But they failed.’’

“But they also failed for another reason,’’ Biden said. “They failed because they continue to fundamentally misunderstand us as they misunderstood us on that day. For the true source of American power does not lie within that building. For as Americans, we draw our strength from the rich tapestry of our people.’’

The New York Times contributed to this report