"Whether they stood up in times of war, signed up in times of peace or were called up by a draft board," Obama said. "They embodied the best of America."
The rows of white gravestones at Arlington "belong to young Americans," he noted. "Those who never lived to be honored as veterans for their service."
Obama also laid a wreath at the cemetery's Tomb of the Unknowns.
He was joined at the ceremony by the country's top military brass, including Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford, who said Memorial Day is "a reminder of the real cost of freedom, the real cost of security, and that's the human cost."
At Section 60 of the cemetery, families and friends of fallen soldiers began gathering early Monday morning to pay tribute to their loved ones.
Their presence serves as a reminder that the U.S. military is still engaged in dangerous efforts across the Middle East.
Nearly 10,000 U.S. service members are stationed in Afghanistan, and the U.S. will maintain a presence there into 2017 despite Obama's efforts to end that mission.
There are also more than 4,000 American troops in Iraq and Syria participating in the anti-ISIS effort in many of the same cities and towns where American lives were lost in the Iraq War.
These troops primarily serve
in an advisory capacity, but have occasionally come under fire.
Three soldiers lost their lives in combat since the U.S. mission there began in the summer of 2014. Most recently, 31-year-old Navy SEAL Charles Keating IV
was killed in a firefight with ISIS fighters earlier this month.
Obama recognized Keating, Staff Sgt. Louis Cardin and Master Sgt. Joshua Wheeler by name Monday and acknowledged the 20 "brave Americans" who have died in combat in Afghanistan since last Memorial Day.
"As commander in chief, I have no greater responsibility than leading our men and women in uniform," he said. "I have no more solemn obligation than sending them into harm's way."
"I think about this every time I approve an operation as president," he added, "every time -- as a husband and father -- that I sign a condolence letter, every time Michelle and I sit at the bedside of a wounded warrior, or grieve and hug members of a Gold Star family."