ARLINGTON, Va.  President Obama observed Memorial Day on Monday just as his predecessors had, by placing a wreath at the Tomb of the Unknowns here.

But Mr. Obama added a twist: he sent a second wreath to a memorial honoring blacks who fought in the Civil War.

“They felt some tug; they answered a call; they said, ‘I’ll go,’ ” Mr. Obama said after the wreath-laying ceremony in a 12-minute address paying tribute to veterans and fallen soldiers. “That is why they are the best of America, and that is what separates them from those of us who have not served in uniform: their extraordinary willingness to risk their lives for people they never met.”

Image President Obama took part in a wreath laying ceremony at the Tomb of the Unknown at Arlington National Cemetery in Arlington, Virginia. Credit... Mandel Ngan/Agence France-Presse — Getty Images

The president asked all Americans to pause at 3 p.m. on Monday for a moment of “national unity” to honor the war dead. “I ask you to ring a bell, or offer a prayer, say a silent ‘thank you,’ ” Mr. Obama said, “and commit to give something back to this nation, something lasting, in their memory.”