President Donald Trump expressed his nation's "boundless and undying" gratitude Monday to Americans who have fallen in battle and to the families they left behind, hailing as heroes the hundreds of thousands buried at Arlington National Cemetery.

In his first Memorial Day remarks as president, Trump told the stories of two soldiers who died in Afghanistan, Green Beret Capt. Andrew D. Byers of Colorado Springs and Christopher D. Horton of the Oklahoma National Guard, as Byers' parents and Horton's widow looked on.

Of them and all who died in military service, he said: "Words cannot measure the depth of their devotion, the purity of their love or the totality of their courage."

Horton, a sniper sent to Afghanistan in 2011, died in a gun battle with the Taliban near the Pakistan border three months into his deployment. Byers was on his third combat tour and, Trump said, ran through smoke and a hail of bullets to rescue an Afghan soldier when he was killed. "We stand in awe of your son and his courageous sacrifice," Trump told David and Rose Byers, the parents.

The president hailed Bob Dole, the former senator who suffered lifelong injuries in World War II and attended Monday's ceremony. And he singled out his homeland security secretary, John Kelly, for whom military sacrifice could not be any closer to home.

Kelly led the U.S. Southern Command and lost his son, Robert Kelly, who stepped on a land mine while on a Marine patrol in Afghanistan in November 2010. John Kelly's other son, Johnny, is preparing for his fifth military deployment. And Kelly's son-in-law Jake is a wounded warrior.

Trump placed a wreath at the Tomb of the Unknown Soldier before his remarks. Afterward, he visited a section of the cemetery where military members killed in the wars in Iraq and Afghanistan are buried. Trump walked among the white marble headstones and greeted families.