(CNN) -- Colin Kaepernick has become larger than football since he refused to stand two years ago as the National Anthem played before NFL games.
An icon in the fight against police brutality and racial injustice, Kaepernick, who hasn't played in the league since the 2016 season, now stars in Nike's new ad campaign. The spot features a black and white close-up of his face with the caption: "Believe in something. Even if it means sacrificing everything."
Here's how we got here:
What was Kaepernick protesting in the first place?
As a quarterback for the San Francisco 49ers, Kaepernick sparked controversy when he sat, then knelt, during the National Anthem before several 2016 NFL preseason and regular-season games. He said he did so to protest police shootings of African-American men and other social injustices faced by black people in the United States.
"To me, this is something that has to change," Kaepernick said in an August 2016 interview. "And when there's significant change and I feel like that flag represents what it's supposed to represent and this country is representing people the way that it's supposed to, I'll stand."
Kaepernick also said he could not "show pride in a flag for a country that oppresses black people and people of color."
After first, Kaepernick sat during the anthem. Later, he opted instead to kneel "to show more respect for men and women who fight for the country." The change came at the suggestion of former NFL player and Green Beret Nate Boyer.
Who else participated?
Kaepernick's protest inspired other players. His teammate, Eric Reid, soon knelt beside him. Other teammates joined, including Antoine Bethea, Eli Harold, Jaquiski Tartt and Rashard Robinson.
Players who sat or knelt during the 2016 season included: Jeremy Lane of the Seattle Seahawks, Brandon Marshall of the Denver Broncos and the Miami Dolphins' Arian Foster, Michael Thomas, Kenny Stills and Jelani Jenkins. Other players, including the Philadelphia Eagles' Malcolm Jenkins, raised fists.
When the 2017 season rolled around, more players joined the protest.
Michael Bennett, then a defensive end for the Seattle Seahawks, told CNN last August he couldn't stand for the National Anthem until he saw equality and freedom.
Twelve members of the Cleveland Browns took a knee last August in their preseason game against the New York Giants. Other teammates huddled around the group in support.
Browns tight end Seth DeValve was identified as the first white NFL player to kneel. DeValve, whose wife is African-American, said he didn't realize he was the first white player to do so.
"I, myself, will be raising children who don't look like me," DeValve said. "I want to do my part, as well, to do everything I can to raise them in a better environment (than) we have right now. I wanted to take that opportunity with my teammates to pray for our country and also to draw attention to the fact that we have work to do. That's why I did what I did."
How did the NFL respond?
After Kaepernick sat for the first time, his team and the NFL released statements saying players were encouraged, but not required, to stand for the anthem, which is played before every professional sporting event in the country.
"The national anthem is and always will be a special part of the pre-game ceremony," the 49ers said in a statement at the time. "It is an opportunity to honor our country and reflect on the great liberties we are afforded as its citizens. In respecting such American principles as freedom of religion and freedom of expression, we recognize the right of an individual to choose and participate, or not, in our celebration of the national anthem."
But things changed in 2017, as Kaepernick became a free agent. No team offered him a contract, and that October, he filed a grievance against the league, accusing team owners of colluding to keep him from being signed.
Athletes who protest peacefully "should not be punished," Kaepernick's attorney said in a statement announcing the grievance.
Still, players continued to kneel. And public backlash mounted until team owners in May declared that all team personnel on the field during the anthem must "stand and show respect for the flag and the Anthem."
Under the policy, players could stay in the locker room during the anthem -- but if any team personnel on the field knelt, the team would be fined. Teams then could decide whether and how to punish offending staff.
That didn't sit well with players. Their union in July filed a grievance, arguing the new policy infringed upon player rights and was enacted without consulting the union.
A week later, NFL officials said they would put the policy on hold so they could discuss a solution with the players association.
How is President Trump involved?
President Donald Trump has not been a fan of players taking a knee.
During his first NFL season in the White House, Trump last September said during a rally in Alabama that NFL owners should respond to players who do so by saying, "Get that son of a bitch off the field right now, he's fired. He's fired!"
In an act of defiance against Trump's statements, players from all 28 teams participated days later in some form of protest, along with many coaches and team owners. Jaguars owner Shad Khan locked arms with players during the anthem as at least a dozen of his players took a knee. The Dallas Cowboys, including owner Jerry Jones, took a knee before locking arms and standing during the anthem. Atlanta Falcons head coach Dan Quinn also locked arms with his players.
Neither Trump nor the players have let up this season. Two Miami Dolphins players knelt last month during the anthem during a preseason game, with Dolphins defensive end Robert Quinn raising his fist during the song. Jenkins, the Philadelphia Eagles captain, and cornerback De'Vante Bausby raised their fists during the song. Several Jacksonville Jaguars were not on the field as the anthem played.
In response, Trump took to Twitter.
"The NFL players are at it again - taking a knee when they should be standing proudly for the National Anthem," the President tweeted August 10. "Numerous players, from different teams, wanted to show their 'outrage' at something that most of them are unable to define. They make a fortune doing what they love."
He continued: "Be happy, be cool! A football game, that fans are paying soooo much money to watch and enjoy, is no place to protest. Most of that money goes to the players anyway. Find another way to protest. Stand proudly for your National Anthem or be Suspended Without Pay!"
What other sports joined in?
Kaepernick's protest had ripple effects across the sports world, something that came to be called the "Kaepernick effect."
In 2016, US soccer international and Seattle Reign midfielder Megan Rapinoe knelt during the National Anthem in a show of solidarity with Kaepernick.
"I think it's actually pretty disgusting the way (Kaepernick) was treated and the way that a lot of the media has covered it and made it about something that it absolutely isn't," Rapinoe said. "We need to have a more thoughtful, two-sided conversation about racial issues in this country."
High school students protested during the anthem as well.
Singer Denasia Lawrence knelt at midcourt as she took the mic in 2016 to sing the anthem at an NBA preseason game between the Miami Heat and Philadelphia 76ers.
NASCAR released a statement in 2017 affirming its respect for the National Anthem and noting it's "always been a hallmark of our pre-race events."
The organization, which had earned praised from Trump, also acknowledged the right to free speech and peaceful protest.
"Thanks to the sacrifices of many, we live in a country of unparalleled freedoms and countless liberties, including the right to peacefully express one's opinion," the statement said, adding that "sports are a unifying influence in our society, bringing people of differing backgrounds and beliefs together."
The WNBA also was the site of protests. Before Game 1 of last year's finals series, the Los Angeles Sparks staged a collective walkout during the anthem -- to a clatter of boos -- as their opponents, the Minnesota Lynx, linked arms while standing. As the anthem played before the next games, the Sparks stayed in the locker room.
What's happened to Kaepernick since his first protest?
Since becoming a free agent, Kaepernick has fulfilled a September 2016 pledge to donate $1 million to organizations working in what he's described as oppressed communities.
Meantime, an arbiter last week denied the NFL's request to throw out his grievance, allowing his case to go to a trial, tweeted his lawyer, Mark Geragos, who is also a CNN legal commentator.
Kaepernick also has been honored numerous times since 2016 for his social justice work. In April, he was named an Amnesty International Ambassador of Conscience, the human rights group's highest honor.
"The Ambassador of Conscience award celebrates the spirit of activism and exceptional courage, as embodied by Colin Kaepernick," Salil Shetty, Amnesty International's secretary-general, said in a statement. "He is an athlete who is now widely recognized for his activism because of his refusal to ignore or accept racial discrimination."
Kaepernick was named GQ's Citizen of the Year for 2017 and accepted the 2017 Sports Illustrated Muhammad Ali Legacy Award. He also received the Eason Monroe Courageous Advocate Award from the American Civil Liberties Union of Southern California.
And in a tweet just days before the 2018 NFL regular-season opener, Kaepernick posted the ad that features him as part of the campaign for the 30th anniversary of the athletic apparel company's iconic slogan, "Just Do It."
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