"Those of you who can stand please do so, those who cannot please raise your hand so that our nation can pay tribute to your service. We thank you," he said to hundreds gathered at the memorial just off the National Mall in Washington, DC.

"If you're an American and you see a veteran, maybe with a prosthetic arm or leg, maybe burns on their face, don't ever look away. Do not turn away," Mr Obama said.

"You go up and you reach out to shake their hand and you look them in the eye and you say those words every veteran should hear all the time: 'Welcome home, thank you, we need you more than ever, you helped us stay strong, you helped us stay free.' Every wounded warrior, every disabled veteran, thank you."

The President drew a burst of applause when he said: "Let's never rush into war, because it is America's sons and daughters who bear the scars of war for the rest of their lives. Let us only send them into harm's way when it's absolutely necessary."

One of those who could not stand was Armando Albarran. Now 68, Mr Albarran was just 18 when he was called up for the Vietnam War and 19 when he lost his legs a few short months later.

The paratrooper remembers mines detonating as his company moved through a jungle valley in a search and destroy mission, and he remembers gripping his M16 as medics tied tourniquets around his upper thighs. He remembers lifting his head and seeing a smoking boot that he thought was probably his.

He did not realise he had lost his legs until his brother-in-law managed call him in a Saigon hospital recovery ward a few days later and asked what his injuries were.

Mr Albarran says he peered beneath his sheet, saw the absence and said into the phone, "I am alive, God saved me, I am alive," before he broke down.

Of his company, 15 were wounded and five killed in the ambush.

"It is the ultimate recognition that you can give a disabled veteran, a memorial of this type," said Mr Albarran of the memorial after the ceremony, noting that at present America is supporting 4 million disabled veterans.

This ground is near sacred to Americans.

In the centre of the National Mall is the Washington Monument, a stone obelisk dedicated to the man who lead the war for US independence. Beyond it is the Lincoln Memorial, which marks the Civil War as much as it does the man who won it. Between the two imposing monuments are memorials to World War II, and the wars in Korea and Vietnam.

Formally known as the American Veterans Disabled for Life Memorial, the new monument was planned in 1997, before the attacks of 2001 and the long wars they precipitated, let alone the renewal of fighting against Islamic State militants.

Asked if was he ever angry about being drafted into an undeclared war that was eventually lost only to lose his legs a month after his arrival, Mr Albarran says he was not.

"Your country calls you for service and you feel somewhat obligated," he said.

He says he is sad for the young men he sees around him at the memorial in baring the scars of later wars. "I'm just hoping that they can overcome their disabilities, and I try to encourage them to do something with their lives, to get to school," he says.

"It is a never-ending process in this country. We have had war all through our history. I guess [it's] the United States that they look up to for help and this is what we do and we are not reluctant, we don't say no, I guess it is just part of the spirit of the United States."