ARLINGTON, Virginia (Reuters) - President Barack Obama honored veterans on Monday by noting "the light of a new day" of having U.S. troops home from Iraq and returning soon from Afghanistan, while promising not to send soldiers back to war without a clear need.

Obama did not mention tension with Iran and Syria in his remarks to veterans and military families at a hot, sunny Memorial Day ceremony, focusing instead on the wars started by his predecessor, George W. Bush, that he wound down as president.

"After a decade under the dark cloud of war, we can see the light of a new day on the horizon," he said at Arlington National Cemetery, drawing applause when he noted the "milestone" of it being the first Memorial Day in nine years without Americans fighting and dying in Iraq.

"As commander in chief, I can tell you that sending our troops into harm's way is the most wrenching decision that I have to make," Obama said shortly after laying a wreath at the Tomb of the Unknown Soldier.

"I can promise you I will never do so unless it is absolutely necessary and that when we do, we must give our troops a clear mission and the full support of a grateful nation."

Later on Monday, Obama will attend a ceremony at the Vietnam Veterans Memorial Wall in Washington.

Defense Secretary Leon Panetta, introducing Obama at the Arlington military cemetery, said that 58,000 Americans died in the Vietnam War. More than 4,000 Americans died in Iraq from 2003 to 2011, and nearly 2,000 have died in Afghanistan from the war's start in 2001 to date.