At Arlington National Cemetery, honoring 150 years of sacrifice can take just about three hours.
That's usually how much time the soldiers of the 3rd U.S. Infantry, known as the Old Guard, need to place hundreds of thousands of American flags at the gravestones and niches of service members interred at the cemetery overlooking the nation's capital from Virginia.
Across the Potomac River, in Northwest Washington, members of the Old Guard also place flags at the graves of those buried at the U.S. Soldiers' and Airmen's Home National Cemetery.
"Our son participated in this today!'' a parent commented on the Old Guard's Facebook page. "So proud!''
There are more than 260,000 gravestones and about 7,300 niches for cremated remains at Arlington, established in 1864, and more than 14,000 veterans are interred at the Soldiers' and Airmen's Cemetery, established in 1861.
The annual "Flags In'' project takes place just before the Memorial Day weekend. It has been a part of the mission of the Old Guard since it was designated the Army's ceremonial unit in 1948, the cemetery's website says.
On Thursday, members of the Old Guard placed their flags under sunny skies and warm spring temperatures. After Memorial Day, the soldiers sweep across the sea of graves once more to remove every flag before the cemeteries open again to visitors.
"I will be there this weekend to see my father,'' a veteran's daughter said in a Facebook post. "Thank you, Old Guard!''