"Democracy itself is in peril, here at home and around the world," Biden warned in an emphatic and emotional address, speaking to military families at Arlington National Cemetery’s Memorial Amphitheater. "How we honor the memory of the fallen will determine whether or not democracy will long endure."
"For those who mourn a loved one today, Jill and I have some idea how you're feeling. Our losses are not the same, but that black hole you feel in your chest as if it's gonna suck you into it, we get," Biden said, noting that Sunday marked the sixth anniversary of his son Beau's death. "I know the incredible pride you felt seeing your loved one wear the uniform of our country and the pride they felt wearing it."
On Friday in Hampton, Virginia, Biden shared details about Beau, who deployed to Iraq in 2008 with the Delaware National Guard. He died at the age of 46 from cancer.
"Our son did not die in Iraq, but he came back -- went as an incredibly healthy young man and came back with a severe brain tumor because his [living quarters] was just downwind from those burn pits -- I don’t know if that’s the reason. But he came home. It was just a matter of how long he lived," Biden said in an address to troops.
His speech Monday focused on the ultimate sacrifice -- U.S. service members dying for their country -- and what he said President Lincoln called "the last full measure of devotion."
He said walking through Arlington's Section 60 -- where many of those killed in Iraq and Afghanistan have been buried -- "reminds me of the cost of war."
"Hundreds of graves here from recent conflicts. Hundreds of patriots gave their all, each of them leaving behind a family who live with their pain and their absence every single day. I want to assure each of those families we will never forget what you gave to our country," he said.
The president announced last month that the U.S. will withdraw all troops from Afghanistan before the 20th anniversary of the Sept. 11 attacks. He said he carries a piece of paper in his jacket pocket with the number of troops killed in Afghanistan and Iraq.
"Today, that number is 7,036. 7,036 fallen angels who have lost their lives in these conflicts and this Memorial Day we honor the legacy and the sacrifice," Biden said. "Duty. Honor. Country. They live for it. They died for it. And we as a nation are eternally grateful."
He went on, "Our freedom and the freedom of innumerable others has been secured by young men and women who answered the call history and gave everything in service of an idea."
The president underscored throughout his speech what American troops have fought for and are still fighting for -- democracy -- or, as he called it, "the soul of America."
"Democracy must be defended at all costs for democracy makes all this possible," Biden said. "Democracy. That's the soul of America. And I believe it's a soul worth fighting for. And so do you, a soul worth dying for."
Biden also mentioned voting rights in his remarks hours after Texas Democrats staged a walkout to block passage of a bill that would have restricted voting rights. It comes as a handful of states have introduced voting bills that advocates have called too restrictive.
"Democracy thrives when the infrastructure of democracy is strong. When people have the right to vote, freely and fairly and conveniently," he said.
"This Memorial Day, remember that not all of us are called to make the ultimate sacrifice. We all are called by God and by history and by conscience to make our nation free and fair," Biden concluded. "If we do our duty, then ages still to come will look back on us and say that we, too, kept the faith."
While leaving Arlington National Cemetery, Biden and the first lady made an unplanned stop to visit a grave at Section 12 -- a primary burial location of those killed and recovered in World War II and the Korean War. They also met with some families.
Ahead of his remarks, Biden participated in a short wreath-laying ceremony at the Tomb of the Unknown Soldier, also known as the Tomb of the Unknowns, with Vice President Kamala Harris and Defense Secretary Lloyd Austin.
Biden on Sunday gave a somber speech in New Castle, Delaware, near his home in Wilmington, at a Memorial Day observance ceremony at Veterans Memorial Park, again paying tribute to Beau.
"If he were here, he would be here as well, paying his respects to all those -- all those who gave so much for our country,” Biden said of his late son.
Comedian Jon Stewart joined lawmakers last week on Capitol Hill to unveil broad legislation that would make it easier for veterans exposed to toxic substances to access Veterans Affairs benefits.
ABC News' Justin Gomez and Ben Gittleson contributed to this report.