Boarding a flight? Active service members go first. Watching a sports event? Then you'll stand and applaud a group of veterans at some point during the game.

America puts its service members on a unique pedestal: they are revered and yet poorly understood.

When Donald Trump insulted the bereaved parents of a soldier killed in Iraq, he broke a US taboo: that troops, veterans and above all their grieving relatives are beyond reproach.

You can disagree with their politics, but you must honor their sacrifice.

Alexander McCoy, 28, a former Marine sergeant studying political science at New York's Columbia University, says he has been upgraded to first and business class, had strangers buy him a beer or pay for his meals, and is offered discounts on movie or theme park tickets simply for being a veteran.

"I do think that America as a culture prizes military service," McCoy told AFP. "Americans kind of mythologize warfare because they have so little experience with it in America."

Unlike many other countries, including Germany where McCoy was stationed, a vast majority of the United States has not experienced war at home since the 1861-65 Civil War.

Other than the Japanese attack on Pearl Harbor attack in 1941 -- more than 2,500 miles (4,000 kilometers) from the mainland -- the September 11, 2001 hijackings were the first major foreign attack on US soil in centuries.

The influence of 9/11, which killed nearly 3,000 people in New York, Washington and Pennsylvania, shattered feelings of safety and bolstered support for the military -- cast as the defenders of America.

Then president George W. Bush quickly sent troops to Afghanistan, and then invaded Iraq. Americans tied yellow ribbons to their vehicles, waved flags and rallied behind the soldiers, holding them above any criticism of the foreign policy behind the wars.

- 'Family business' -

"The Bush administration, and the federal government more broadly, cultivated a kind of popular nationalism after 9/11 for all kinds of reasons," said Michael Allen, a history professor at Northwestern University.

"Bush relied on his gut," Allen said. "How to publicly sell the war on terror -- that was where he really excelled, because it was instinctive for him," he said. "It was very natural for him to rally around the flag."

But the military has not always been so venerated. Opposition to the Vietnam War was so vehement that veterans were treated abominably despite 25 percent of the troops having been drafted and the deaths of more than 58,200.

Yet if veneration has increased towards today's all-volunteer force, in which less than one percent of the population currently serve, so too has a profound disconnect with the civilian world.

US military bases -- once scattered all over the country -- have been consolidated. Many who serve come from a military background, which McCoy believes is creating a "family business" -- almost "a warrior caste."

"People respect vets," says Deborah Gahm, an accountant from Phoenix, Arizona, whose husband has served for more than 20 years, including in Somalia, Iraq and Afghanistan.

"But I don't think it's a deep level of respect, I think it's very much on the surface. In many respects, it's just lip service," she said.

There has been also chronic under-investment in programs that help soldiers transition back into civilian life. Congress is considering major cuts to the educational and housing allowances offered to vets.

"When we make promises to people that sign a contract that could get them killed, we ought to keep those promises. So there are people who like to say that they support our troops but then they don't," Gahm said.

- Damaging stereotypes -

Gahm says she is troubled by the Hollywood portrayal of returning warriors as violent or struggling with post-traumatic stress disorder (PTSD), and believes the public too often treats veterans as victims.

"I think there are really damaging stereotypes out there," she told AFP. "Even vets who have PTSD rarely do violent acts," she added.

While her husband was in Iraq, she once went out to eat with their daughter, and women at the next table were accusing soldiers of signing up to murder, she said. She asked to be moved to shield her daughter from the comments.

Allen sees the roots of today's rituals honoring vets in Richard Nixon's attempts to sideline the anti-Vietnam war movement by identifying with conventional, conservative America and using sport to do so.

He would "obsessively" call attention to his watching of football and bring returning prisoners of war to sporting events, Allen said. POWs were given lifetime free passes to Major League baseball games.

Reassessment of the damage done to veterans by the anti-Vietnam backlash has seen Democrats join Republicans in adopting trenchant support for the military.

"The lesson learned by the political left was you must never criticize soldiers and veterans, it's absolutely forbidden -- to some degree, always the attitude on the political right," said Allen.

"You can't go around criticizing a Gold Star family. That's really very taboo to do," said Gahm of Trump's laceration of Khizr and Ghazala Khan, the parents of a Muslim soldier who was killed in a suicide bombing in Iraq.