San Francisco 49ers quarterback Colin Kaepernick's decision to take a knee during the national anthem has set off a wide-ranging debate over free speech and how far athletes should go with their protests.

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It also has put the nation’s most popular sports league, which regularly wraps itself in the flag, at the center of a debate over what it means to be patriotic — all coinciding with the 15th anniversary of the deadliest terrorist attack on American soil.

Kaepernick, who used to be one of the most dynamic players in the National Football League (NFL), re-entered the spotlight as one of its most controversial over his use of the "Star-Spangled Banner" as a platform to protest against police brutality and racial inequality.

His actions earned him plaudits during an era when many athletes have shied away from voicing their political opinions.

But many others see his protest as inappropriate and an insult to military service members, to whom honoring the flag is sacrosanct.

NFL Commissioner Roger Goodell, the son of a former U.S. senator, chose his words carefully when he broke his silence on Kaepernick this week.

“I don't necessarily agree with what he is doing,” Goodell told The Associated Press Wednesday, while adding that the quarterback has the right to voice his opinions.

His cautious comments are a sign of the vast divide between the league’s owners and players, as well as its liberal and conservative fans, over the anthem protest.

Kaepernick won't actually take the field until Monday night when his team opens the season agains the Los Angeles Rams.

But all eyes will be on the rest of the league as the media and fans watch to see who follows his lead and picks up the protest.

Denver Broncos linebacker Brandon Marshall took a knee during the anthem before the NFL’s first regular-season game on Thursday night.

The Seattle Seahawks are considering a teamwide protest before their game Sunday against the Miami Dolphins.

Seahawks wide receiver Doug Baldwin argued there’s no better time for such a protest than the 9/11 anniversary, despite the controversy it will undoubtedly generate.

“Even if it wasn’t Sept. 11, the point of the protest is to get people to think,” he told The Seattle Times.

“I think it’s very ironic to me that 15 years ago on Sept. 11 was one of the most devastating times in U.S. history and after that day we were probably the most unified that we have ever been,” he said. “And today we struggle to see the unity.”

The heated debate over Kaepernick is a microcosm of the country’s politics, which have become deeply polarized over the past decade and a half, washing away a spirit of national unity that followed the Sept. 11, 2001, attacks.

The topic has consumed sports talk shows on television and radio, and fans of all stripes are weighing in.

Fifty-seven percent of Americans disapprove of Kaepernick’s protest, while 32 percent agree, according to a YouGov poll released this week.

But there are significant racial and partisan divides over Kaepernick. Almost 7 in 10 whites do not approve of his actions, while nearly three-quarters of blacks approve. Fifty-five percent of Democrats approve of the protest, compared to 85 percent of Republicans who disapprove.

The 28-year-old signal-caller’s jersey is now the top seller in the league, even though he’s now a bench player. He’s promised to donate proceeds from his jersey sales to charity.

The nation’s most prominent politicians have been drawn into the debate, and their responses have predictably fallen along partisan lines.

Republican presidential nominee Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE said Kaepernick should leave the country if he won’t stand for the anthem.

“I think it’s a terrible thing, and, you know, maybe he should find a country that works better for him. Let him try; it’s not gonna happen,” he said during a radio interview last week.

Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE hasn’t weighed in on Kaepernick’s protest, even though the quarterback has sharply criticized the Democratic presidential nominee for her use of a private email server as secretary of State and her 1990s description of black teens in gangs as “super predators.”

President Obama defended Kaepernick for “exercising his constitutional right to make a statement,” even though he acknowledged the nature of the protest could drown out his underlying message.

“When it comes to the flag and the National Anthem, and the meaning that that holds for our men and women in uniform and those who fought for us, that is a tough thing for them to get past to then hear what his deeper concerns are,” he said during a news conference in China.

Absent from the politicians’ responses is why the anthem is played before sporting events in the first place.

Sports stadiums and arenas are some of the few public places where the "Star-Spangled Banner" is played on a regular basis.

The practice dates back to baseball games in the mid-19th century, decades before the song officially became the national anthem. It finally caught on in 1918, when it was played at the World Series taking place during World War I.

In the years since, the anthem has become inseparable from American sporting events. And grand patriotic displays have become even more commonplace since 9/11.

The New York Yankees began the practice of playing “God Bless America” during the seventh-inning stretch following the attacks. Teams routinely honor military personnel, police and firefighters by including them in flag displays and on-field reunions with their loved ones.

Vice President Biden is planning to mark the 15th anniversary of 9/11 by attending a ceremony honoring first responders during the anthem before Sunday’s Philadelphia Eagles game, according to his office.

Of course, Kaepernick is hardly the first athlete to protest the anthem — U.S. Olympians Tommie Smith and John Carlos raised their fists as the song was played at a medal ceremony at the 1968 games in Mexico City.

But his actions have caused some to question the meaning behind outward shows of patriotism at professional sporting events.

It’s a sensitive subject for the NFL, which had to repay taxpayers more than $720,000 earlier this year after teams accepted payments from military branches for color guards and other tributes.

League officials are defending the value of playing the anthem before games.

“We believe very strongly in patriotism in the NFL,” Goodell said.

But others are saying it’s time for the anthem to be retired from the games.

“At its worst, the anthem is used as both an ideological cudgel and as a cynical marketing ploy,” wrote Deadspin columnist Drew Magary. “It’s a cheap, easy way for sports franchises to make themselves as unassailable as the song itself.”