President Bush laid a red, white and blue-flowered wreath on the Tomb of the Unknowns (search) on Monday as he vowed to honor the nation's war dead by pursing peace and freedom throughout the world.

"The names of the men buried there are known only to God but their courage and sacrifice will never be forgotten by our nation," Bush said in remarks after a ceremony at Arlington National Cemetery (search), which featured songs like "The Star Spangled Banner" and "Taps."

They "answered the call of service in their nation's hour of need. They stood to fight for America's highest ideals. And when the sun came up this morning, the flag flew at half staff in solemn gratitude and a deep respect," the president added.

Describing America as a "reluctant warrior," Bush remembered the men and women who fought in wars past, adding that today, "another generation is fighting a new war against an enemy that threatens the peace and stability of the world."

The U.S. military is "standing directly between our people and the worst dangers in the world and Americans are grateful to have such brave defenders."

He added: "We must honor them by completing the mission for which they gave their lives; by defeating the terrorists."

Peace — More Than Victory on the Battlefield

Monday's ceremony was the 137th observance of Memorial Day (search) at Arlington Cemetery, where two American presidents, John F. Kennedy and William Howard Taft, are buried.

Interred in the Tomb of the Unknowns are the remains of unidentified servicemen from World Wars I and II, and the Korean War. A Vietnam War casualty was removed from the tomb in 1998, after he was identified using DNA technology.

Before Bush took the stage, Joint Chiefs of Staff Gen. Richard Myers (search) hailed the nation's war veterans for enduring the "long struggle for freedom" and said today's men and women in uniform are likewise fighting for America's freedom in the War on Terrorism.

America's armed forces have the "resolve to continue this difficult fight for as long as it takes," Myers said. "They know that failure is not an option."

"Our armed forces and our coalition partners have defeated two brutal regimes, bringing hope to millions who only knew tyranny and fear ... [but] we still have much work to do," he continued, referring to how the U.S.-led coalition has toppled the oppressive Taliban in Afghanistan and Saddam Hussein's regime in Iraq. "Victory on the battlefield is but a step on the road to peace."

Upwards of 1,650 Americans have died in Iraq since the U.S. led invasion there are over two years ago, with another 12,000 injured.

Although the War on Terror has brought "great costs" to those who have lost loved ones in Afghanistan and Iraq, with those regimes gone, "freedom is on the march and America is more secure," Bush said, receiving a round of applause.

Defense Secretary Donald Rumsfeld (search) told the families of fallen servicemen and women to take comfort in knowing that their sons and daughters died for their country's freedom.

"There are no sufficient words of comfort. The emptiness will linger and it's hard but we can be grateful for the time we had with them, we can celebrate who they were and how they lived their lives … [and take solace in knowing that] those lives were lost in the struggle dedicated to the eternal truth of freedom and the eternal spirit," he said.

"Our country was founded on that spirit and Americans have nurtured it through every war in every era."

In introducing Bush, Rumsfeld said Bush understood the new kind of war America is facing when he looked at a "Pentagon in flames" after terrorists crashed one of four commercial airliners into the Defense Department on Sept. 11, 2001.

The president understands that "liberty's survival here depends on its advance abroad," Rumsfeld added, saying that Bush has promised "to meet violence with patience justice."

Cities across the nation were marking Memorial Day with parades, official ceremonies and solemn visits.

The Memorial Day parade in Washington will feature one of the country's oldest veterans. Lloyd Brown was 16 years old when he signed up to fight in World War I. He's now 103 and one of an estimated 30 U.S. veterans of World War 1 still alive.

Elsewhere, many cities across the nation will be holding their own Memorial Day parades, honoring veterans and those still serving in Iraq, Afghanistan and other spots around the globe.

War Heroes: 'Live Free'

The president's tribute at Arlington came in sharply different circumstances from the Memorial Day visit Bush made to the cemetery's Tomb of the Unknowns two years ago, just as the nation's problems stemming from the Iraq war were becoming apparent.

Before his Memorial Day remarks in 2003, Bush had declared major combat operations at an end, the U.S. government confidently predicted that weapons of mass destruction would be found and American generals said troops were in the process of stabilizing Iraq.

At that time, some 160 American soldiers had been killed in Iraq.

On Monday, Bush evoked the memories of American soldiers who have died, reading excerpts from the letters they wrote, in some cases letters that were to be opened only in the event that the soldier didn't make it home.

"My death will mean nothing if you stop now," Louisiana National Guard Sgt. Michael Evans wrote in a letter home. Evans died Jan. 28 while on patrol in western Baghdad, part of a major security operation to protect the first free Iraqi elections in more than 50 years.

"I know it'll be hard, but I gave my life so you could live, not just live, but live free," Evans wrote.

Bush's nine-minute address was punctuated eight times by applause from a crowd of military families, some of whom were accompanied by soldiers in wheelchairs recovering from their wounds.

As he has done since U.S. forces invaded Iraq in March 2003, the president said the war is part of a greater conflict necessitated by the Sept. 11 terrorist attacks that claimed nearly 3,000 lives, toppled the World Trade Center and heavily damaged the Pentagon.

But it was excerpts from the letters that Bush read to the audience that drew tears from some, along with the strongest applause.

"Realize that I died doing something that I truly love for a purpose greater than myself," Marine Capt. Ryan Beaupre of St. Anne, Ill., wrote to his family. Beaupre was killed when the helicopter he was helping to pilot crashed in Kuwait in the early hours of the war.

Elsewhere in the United States:

— On the beach in Santa Monica, Calif., the anti-war group Veterans for Peace set up a temporary "Arlington West" display of more than 1,600 white crosses in memory of American soldiers killed in Iraq. Some crosses were decorated with flowers along with pictures and names of the dead.

— In Boston, about 70 musicians gathered on Boston's City Hall Plaza for a Memorial Day tribute to civilians killed in Iraq. Performers with wind, percussion or string instruments formed a circle and sounded notes for each of the war's civilian casualties — a high note for a child, a medium note for a woman and a low note for a man, up to 25,000 notes in all.

"The civilian deaths are not taken into account and rarely reported," said Nancy Adams, director of the Mobius Artists Group, which organized the event. "We're trying to take note of all the causalities of war."

— Also Monday, a memorial for American soldiers killed in Iraq and Afghanistan was dedicated in a courtyard behind the Old North Church in Boston's North End.

"We do this to remember," said the Rev. Patricia Handloss of the Old North Church. "It is important we never forget what our young men and women are doing, not for us but for the world."

FOX News' Liza Porteus and The Associated Press contributed to this report.