Kansas City Chiefs cornerback Marcus Peters raises his fist in the air during the national anthem on Sunday. (John Sleezer/AP)

In Jacksonville, Fla., football fans chanted “U-S-A!” under a searing midday sun. In Kansas City, Mo., a lone, defiant fist punched the air. In Philadelphia, players helped hoist a massive flag — together — and in Seattle, a team locked arms — together.

All across a football-loving nation, patriotism preceded pigskin Sunday, the opening weekend of the NFL’s new season, and fans and players alike wrestled with the disparate, nuanced ideas of nationalism, dissent and loyalty — often all three at the same time.

Upset over social injustice and police violence, San Francisco 49ers quarterback Colin Kaepernick opted not to stand for the national anthem during the preseason. At first, no one noticed. The weeks since, though, have seen wide-ranging conversations, bridging politics and sports, and Sunday’s slate of NFL games showed that Kaepernick’s once-silent action suddenly has a bit of volume behind it.

“We live in the greatest country in the world because we have taken challenges head on,” Seattle Seahawks wide receiver Doug Baldwin explained in a weekend Facebook message. “We have made the uncomfortable the norm. On one of the most memorable days in US history, we have an opportunity to unite again for a cause. To show why our country is so great.”

Sunday also marked the anniversary of the 9/11 attacks, a time when patriotism and all its symbols and pageantry are already on full display. On the stadium video scoreboards and network pregame shows, President Obama appeared in a prerecorded message, saying: “It’s Sunday here in America. That means it’s time for football. But on this day 15 years ago, the world was shaken.”

The Washington Post spoke to football fans at a preseason tailgate about the role of politics in football and this is what they said. (Dalton Bennett/The Washington Post)

The confluence of events meant patriotism took on multiple forms: pride in the military, servicemen and servicewomen, as well as the flag and the country it represents; and players exercising the rights and freedoms afforded to them, the military and servicemen and servicewomen, also symbolized by the same flag.

“I think it’s all connected,” Seattle Coach Pete Carroll said last week, after his players decided to lock arms in solidarity, “and I think understanding that is very important, and they have. They’ve taken all that into account.”

In many cities, the anthem played just as any other Sunday in any other season: heads bowed, lips mouthing the words, some eyes closed, some hands over hearts. But there were signs of silent protest in a handful of stadiums.

Some members of the Miami Dolphins, including running back Arian Foster, defensive back Michael Thomas, linebacker Jelani Jenkins and wide receiver Kenny Stills, stood solemnly during a pregame 9/11 ceremony in Seattle, then knelt and placed a hand over their hearts during the playing of “The Star-Spangled Banner.” The Dolphins said in a statement, “We hope today’s events will continue a respectful and thoughtful dialogue in our community on unity, inclusiveness and togetherness.”

Earlier in the day, Chiefs players in Kansas City also stood with their arms locked. At one end, cornerback Marcus Peters held his right fist high in the air while the anthem played.

“It was our goal to be unified as a team and be respectful of everyone’s opinions, and the remembrance of 9/11,” the Chiefs players said in a statement. “It’s our job as professional athletes to make a positive impact on our communities and to be proactive when change is needed.”

An overall view of the field at CenturyLink Field as the Seattle Seahawks face the Miami Dolphins in Seattle. (Jonathan Ferrey/Getty Images)

Kaepernick ignited a massive and polarized debate — between those who believe he’s disrespecting the country and others who believe he’s calling attention to an important social issue. Women’s soccer star Megan Rapinoe similarly followed suit and similarly received a backlash of criticism, and football players across the league spent several days debating what — if anything — to do on the NFL’s opening weekend.

“I feel like everybody has different things that they believe in,” Baltimore Ravens wide receiver Mike Wallace said Sunday. “If you feel like you need to do that, do it. This is America, land of the free, so you can do that if you want to, and I don’t think anybody should have a problem with it.”

But many do. In Baltimore on Sunday, up in Section 142, fan Debbie Siegman of Littlestown, Pa., held a sign that read, “9-11-01 Never Forget,” as she watched the Ravens play the Buffalo Bills.

“I understand protesting Black Lives Matter, but that has nothing to do with football,” she said. “All lives matter.”

Historian Gary Gerstle said Sunday’s actions by NFL players are just the latest in a trend of professional athletes speaking up on social issues.

“I think for a long time, the model was Michael Jordan — do your job, become the best, don’t let politics intrude on your sport,” he said. “I think what we’re seeing now is that coming apart. This is not seen as an isolated event but something that’s happened over the past year, with Ferguson being an inflection point. It’s a rethinking by a lot of black athletes about the proper decorum.”

While there’s a long history of athletes waging public protest, from Muhammad Ali to Tommie Smith and John Carlos, professionally athletes have increasingly become vocal in recent years, particularly when they sense social injustice or inequality. NBA players such as Carmelo Anthony and Dwyane Wade have spoken out against police violence, and last year the University of Missouri football team threatened to boycott a game because of race relations on campus.

Gerstle, the Paul Mellon Professor of American History at the University of Cambridge, said the actions by athletes Sunday aren’t necessarily anti-American; they can be viewed, in fact, as a vital form of patriotism. He said patriotism has traditionally taken on multiple forms in the United States.

“There are people who say your first obligation as an American citizen is stand with the flag, stand with military, stand for the anthem,” he said. “Declare your patriotism and then we can talk about what we need to change. In times of war it becomes more difficult to carve out a space for that other form of patriotism because there are those who say we must defend the flag first and foremost.”

NFL players have wrestled with how to protest without offending, how to make a point without being disrespectful, particularly on a revered day such as Sept. 11.

Keenan Reynolds is a member of the Ravens’ practice squad who played quarterback for four seasons at Navy. “We fight to protect your freedom to do that, regardless of how I feel about it. How I feel about it is irrelevant. The flag gives you the right to do that,” Reynolds said Saturday.

Protest wasn’t at the forefront of every player’s mind. As the anthem played in Philadelphia, players for the Eagles and Cleveland Browns stood alongside military representatives, uniformed personnel and Vice President Biden to hold a giant American flag that nearly covered the entire field.

Browns wide receiver Terrelle Pryor said his team’s approach to the anthem, with holding the flag, was not planned beforehand.

“We have a free world, and I think you have to give appreciation to guys and appreciate the life that we have, that we can walk free,” he said. “We do what we want within the rules. It was awesome for us to go out there and hold the banner with those guys.”

The league opened its season Thursday with a game between the Denver Broncos and the Carolina Panthers. Denver linebacker Brandon Marshall took a knee during the anthem and became the first player to stage an anthem protest in the regular season. The next morning he became the first player to lose an endorsement deal because of his actions.

“While we respect Brandon’s right of expression, his actions are not a representation of our organization and membership,” a spokesman for Air Academy Federal Credit Union said.

Kaepernick didn’t intend to start a movement. He sat during the anthem in the 49ers’ third preseason game and knelt during the fourth. He didn’t explain his protest until a reporter noticed him on the sideline and inquired. But it has caught on and spread. At recent high school games, anthem protests have been spotted in Virginia, Illinois, Nebraska, Minnesota and Maryland.

The players at Watkins Mill in Montgomery County approached their coaches about doing something and were encouraged to think through the issues. So they had discussions and did research, studying the lyrics and history of “The Star-Spangled Banner” and talking about what it represents.

“It’s just surfacing more now that the NFL players are doing that,” said Reggie Spears, the Watkins Mill athletic director. “I’m sure it’s starting to trigger some discussion among the players and their support behind the protest and exercising their rights.”

On Friday night several Watkins Mill players decided to take a knee during the anthem. “We just wanted to make a statement,” junior quarterback Markel Grant said, “that America is not what you think it is.”

But even at the high school level, the protest incites passion on both sides. After the Watkins Mill players took a knee Friday, a player from Damascus, their next opponent, tweeted: “Watkins Mill can disrespect my country all they want. But there’s going to be a price to pay if they do it on my field.”

Post reporters Mark Maske; Jacob Bogage, reporting from Baltimore; Gene Wang; and Eric Goldwein contributed to this report.