The pressing question for many viewers tuning in to a preseason football game on Thursday night will be whether San Francisco 49ers quarterback Colin Kaepernick will once more refuse to stand for the national anthem.

But that question obscures some more meaningful ones:

Why is the national anthem a staple of sporting events to begin with? Why does the United States stand apart in making the anthem a part of the pregame ritual? And what does it mean to be patriotic?

Kaepernick, once regarded as one of the N.F.L.’s top players, has suddenly become its most provocative ahead of Thursday’s game in San Diego because, in a country that is unusual in its marriage of sports and public patriotism, he has chosen the anthem as the moment to communicate a message of protest.

After declining to stand during the anthem before a game on Friday night, Kaepernick explained that he was motivated by issues of police brutality and racial injustice in the United States — eliciting vitriol from fans who believed his gesture was an affront and praise from those who admired his decision to take a public stand at a time when few prominent athletes are willing to do so.