Charles Clymer is a six-year Army veteran. A native Texan. A dyed-in-the-wool Dallas Cowboys football fanatic who played defensive end in high school and hangs a Texas Longhorns hat on the bedroom wall, next to an oversized American flag.

The 30-year-old also resents U.S. President Donald Trump's call to fire players who choose to kneel in protest of systemic racial oppression during the national anthem. The president's remarks crystallized a complicated relationship with what Clymer calls "the most political institution in the country."

No, not Congress or the White House. The National Football League.

On Monday night, the Cowboys became the latest team to kneel in solidarity, arms linked with team owner Jerry Jones. As a chorus of boos rained down at the University of Phoenix Stadium, Clymer felt "damn proud" of the team.

"I have my Dak Prescott jersey in my closet. I have an American flag displayed in my apartment pretty prominently," Clymer said. "But there's this narrative that patriotism is linked to football and the grand history of America, and everything the NFL does is tied to patriotic pageantry, with every game starting out with the colours being presented, the anthem, the enormous field-sized flag that's furled out."

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U.S. Army veteran Charles Clymer, a diehard Dallas Cowboys fan, poses in a Cowboys jersey against an American flag. (Courtesy Charles Clymer)

The sport's connection with pro-U.S. sentiment intertwined with military service is "absurd" to the George Washington University graduate student.

"I love football. But it leans far, far to the right. It's a conservative culture. And I resent the way any sort of criticism of the NFL or how America has its problems with its culture is unpatriotic."

Pregame military flyovers, "salute to service"-branded NFL merchandise and combat-referencing plays about blitzes and ground attacks may have helped to conflate the country's most popular sport with love of country and the armed forces.

Unlike the National Basketball Association — which opened conversations about Black Lives Matter and police shootings of unarmed black men — the NFL has in the past shied away from overt political discourse, dragging its heels to weigh in on debates over the sport's challenges with traumatic brain injury and domestic violence.

But not this time. Not after Trump's remarks on Friday calling players who take a knee during the anthem unpatriotic "sons of bitches."

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The Dallas Cowboys players, coaches and staff take a knee prior to the National Anthem before the game against the Arizona Cardinals at University of Phoenix Stadium on Monday. (Matt Kartozian/Reuters)

The irony is that after Trump's remarks on Friday, the activism "became a much bigger thing," said Samuel Freedman, author of Breaking the Line, an account of the desegregation of college teams.

"Trump was so dismissive, so profane, so caustic. The fact he threatened players with the loss of their livelihoods for exercising their constitutional rights, people weren't going to be intimidated so easily," he said.

"That's part of why you saw so many players — far more than had taken a knee or a clenched fist before."

Skewing conservative

Football has long identified strongly with conservative-skewing Americans, likely buoyed by Southern powerhouse college programs in deep-red states and at least $5.4 million paid out to 14 teams from 2011-2015 by the Department of Defense for elaborate "paid patriotism" tributes to the troops.

Trump bashes NFL players as protests grow | 'This has nothing to do with race'2:46

A 2014 Experian Simmons study found that registered Republicans were 21 per cent more likely to be NFL watchers than Democrats. And the Sunlight Foundation in 2011 found that contributions from the league's 32 teams were nearly three times more for Republicans than for Democrats.

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Buffalo Bills players kneel during the playing of the national anthem prior to an NFL game against the Denver Broncos on Sunday. (Jeffrey T. Barnes/Associated Press)

Freedman said that although displays of activism are often viewed as rare in the NFL, the idea that the league occupies some politics-free zone is a fallacy. Football, he argues, has long had a role in perpetuating racial inequality with its history of segregated college teams.

"There's been politicization of football all along, but on the part of political and social conservatives that ignores that fact, and only gets upset when the activism is happening from the political left," Freedman said.

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A unified message

Which might have made it all the more surprising last Sunday. At games nationwide, NFL players, owners and coaches sent a unified message to Trump, defending players' rights to peaceful protest.

It began early at London's Wembley Stadium, when the Baltimore Ravens and Jacksonville Jaguars locked arms on the sidelines, some taking a knee, during the Star-Spangled Banner.

As the day progressed, the majority of the Oakland Raiders knelt during the anthem or linked arms before their game against the Washington Redskins. Several Redskins players took a knee while most stood arm in arm. Except for one player, who later explained he had made a "mistake," the Pittsburgh Steelers skipped the anthem together, as did the Tennessee Titans. The Seattle Seahawks followed their example by staying in the locker room.

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U.S. President Donald Trump speaks to reporters upon his return to the White House in Washington on Sunday, a weekend that began with little hint that the president would reignite the issue of protests during the playing of the anthem at sporting events. (Mike Theiler/Reuters)

Even Trump endorsers such as New England Patriots owner Bob Kraft and former Buffalo Bills coach Rex Ryan came out to denounce the president.

"Trump forced their hand," said David Karpf, an associate professor who studies political communication and media at George Washington University, noting that former San Francisco 49ers quarterback Colin Kaepernick began the "take a knee" movement last year as an act of solo protest.

"I am not going to stand up to show pride in a flag for a country that oppresses black people and people of colour," Kaepernick explained in a written statement last year. "To me, this is bigger than football."

Kaepernick was considered to have been blackballed by the league, failing to sign with any team in the 2017 season. When the issue was solely about him and the small set of players rallying to his side, Karpf said, "it was easy for most of the league to stay out of it."

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Former San Francisco quarterback Colin Kaepernick, centre, remains a political lightning rod for his refusal to stand for the American national anthem. (Marcio Jose Sanchez/Associated Press)

"And then Donald Trump calls them 'sons of bitches,' and the question is put before every single player: Are you going to stand with a guy who, whether you agree with the style of protest he chose, he could be on your team? And the president just called him a 'son of a bitch' because he exercised his right to peaceful protest?"

As for whether the protest would continue, Kenneth Shropshire, who has written about race and sports in the U.S. and runs the Global Sports Institute at Arizona State University, told the New York Times the effectiveness of the activism may depend on how long the movement lasts.

It may well be that part of the anger on the part of NFL management comes from the president meddling in their corporate affairs, he told the Times.

"It'll be interesting if some owner signs Kaepernick now, just to show that Trump can't engage in their business," he said.