ARLINGTON, Va. - President Bush on Monday honored US troops who have fought and died for freedom and expressed his steely resolve to succeed in the war in Iraq. "As before in our history, Americans find ourselves under attack and underestimated," he said.

President Bush, left, lays a wreath at the Tomb of the Unknowns, marking Memorial Day, Monday, May 28, 2007, at Arlington National Cemetery in Arlington, Va. [AP]

Bush marked his sixth Memorial Day as a wartime president with a somber speech at Arlington National Cemetery. He said he hoped the United States will always prove worthy of the sacrifices fallen troops have made, and recognized the grief suffered by families and friends of troops killed in war, most recently in Afghanistan and Iraq.

"Now this hallowed ground receives a new generation of heroes - men and women who gave their lives in places such as Kabul and Kandahar, Baghdad and Ramadi," he said. "Like those who came before them, they did not want war, but they answered the call when it came. They believed in something larger than themselves. They fought for our country, and our country unites to mourn them as one."

The president's motorcade was greeted at Arlington by scores of tourists who waved at Bush. Just before his limousine crossed over the Potomac River into Virginia, a man held up a sign saying, "Bring our troops home."

Members of the armed forces carrying rifles fitted with bayonets stood at attention as Bush's motorcade wended its way through rows of white tombstones marked with tiny American flags. Some soldiers were astride horses that flinched when canons were fired, sending bluish white smoke over the cemetery.

Bush spoke under overcast skies at a marble amphitheater after he laid a wreath of red, white and blue flowers at the Tomb of the Unknowns and stood, his hand over his heart, during a drum roll and the playing of Taps. First lady Laura Bush watched the ceremony with relatives of fallen troops.

Those who have died in the Civil War, World War I, World War II and Vietnam conflicts fought for freedoms, which come at great cost and will survive only as long as there are Americans willing to defend the nation against determined enemies, he said.

"Our enemies long for our retreat," Bush said. "They question our moral purpose. They doubt our strength of will. Yet even after five years of war, our finest citizens continue to answer our enemies with courage and confidence."

He noted that 174 Marines - nearly one-fourth of a battalion - recently asked to have their enlistments extended.

"Those who serve are not fatalists or cynics," Bush said. "They know that one day this war will end as all wars do. Our duty is to ensure that its outcome justifies the sacrifices made by those who fought and died in it.

"From their deaths must come a world where the cruel dreams of tyrants and terrorists are frustrated and foiled - where our nation is more secure from attack, and where the gift of liberty is secured for millions who have never known it."

At least 3,452 members of the US military have died since the beginning of the war in Iraq in March 2003, according to an Associated Press count. At least 325 members of the US military have died in Afghanistan, Pakistan and Uzbekistan as a result of the US invasion of Afghanistan in late 2001, according to the Defense Department.