The United States as a free people and free society remain thanks to the 'dear cost' of soldiers who have given their lives, President Barack Obama said on Monday at Arlington National Cemetery at a Memorial Day Service.

We remember that the blessings we enjoy as Americans came at a dear cost; that our very presence here today, as free people in a free society, bears testimony to their enduring legacy, Obama said.

Obama was in Arlington, Virgina, where he attended a morning Memorial Day service. He also visited a section of the cemetery set aside for those soldiers who have died since September 11, 2001.

Earlier he visited the 'Tomb of the Unknowns' and presented a wreath of red and white flowers.

Obama said Americans can and must honor the sacrifice of fallen soldiers by holding their memories close to our hearts, and heeding the example they set.

He said the U.S. would keep searching for missing soldiers and those held as prisoners of war, as well as serving them as well as they serve us  -- from the moment they enter the military, to the moment they leave it, to the moment they are laid to rest.

Obama recalled soldiers who fell in the Revolutionary War, Civil War and battlefields of our times.

Citing members of a family who mailed him a story of their son, Obama quoted a verse from the biblical Book of Isaiah where the prophet says he has been called to deliver stern words to his people.

When I heard the voice of the Lord saying, 'Whom shall I send? And who will go for us?' And I said, 'Here I am Send me!

That's what we memorialize today.  That spirit that says, send me, no matter the mission.  Send me, no matter the risk.  Send me, no matter how great the sacrifice I am called to make, Obama said.

It's natural, when we lose someone we care about, to ask why it had to be them, he said.

These are questions that cannot be answered by us. But on this day we remember that it is on our behalf that that they ... gave their lives.

Obama recalled a pair of friends that were more like brothers that met at the U.S. naval Academy. One was killed by a sniper in Iraq in 2007 while attempting to rescue his fellow Marines from danger. The other deployed to Afghanistan last year and gave his own life, along with eight others in a helicopter crash.

Their families decided to move one of the friends from his grave in Pennsylvania to lay him in a grave next to his friend at Arlington.

Warriors for freedom, reads the epitaph, brothers forever.

Their friendship reflects the meaning of Memorial Day. Brotherhood. Sacrifice. Love of country. And it is my fervent prayer that we may honor the memory of the fallen by living out those ideals every day of our lives, in the military and beyond.  May God bless the souls of the venerable warriors we've lost, and the country for which they died.