MINNEAPOLIS -- President Trump's call for NFL owners to fire players who take a knee during the playing of the national anthem sparked a renewed partisan debate, and drew a strong response from players and owners who stood Sunday in solidarity against the president's words.

The controversy also prompted some to ask how Francis Scott Key's song, The Star-Spangled Banner, became so inextricably linked to the NFL and all other professional sports.

There's no law or NFL rule requiring the national anthem to be played at games. And, contrary to popular belief, players aren't required to stand while it's being played.

The league's internal operations manual, which is not published, requires players to be on the field while the anthem is played. It encourages them -- but doesn't require them -- to stand while the song is being played or sung. The same manual bars players from talking to each other during the performance of the anthem.

TV networks often go to commercial breaks during the anthem rather than including it in the broadcast. But fans have long been accustomed to hearing the song and seeing players stand at attention.

When did it all start?

Some historians have found evidence of The Star-Spangled Banner being played at sporting events as early as the 1890s, but the first extensive use came during the 1918 World Series between the Chicago Cubs and the Boston Red Sox.

World War I was raging in Europe, and American troops were by then fully involved in the conflict. On Sept. 5, 1918 Babe Ruth, still a Red Sox pitcher at the time, came to Chicago for the World Series. The home team Cubs had a band play the anthem during the 7th Inning Stretch.

The Red Sox, as legend has it, moved the playing of the Star-Spangled banner to the beginning of the game when the series moved to Boston. And, keep in mind, the song wouldn't be adopted officially as the national anthem until 1931.

Fast forward to 1945, as World War II was winding down, NFL Commissioner Elmer Layden hosted President Harry S. Truman at a Washington Redskins game. The Star-Spangled banner was played at that game, and Layden said it should be played at all NFL games to sustain fans' patriotic fervor in the postwar years.

By the 1960s the playing of the national anthem had become fairly universal, while some sports teams also added God Bless America to the musical mix.

Since the first Gulf War in 1990 the opening ceremonies of pro sports contests have taken on more of a military theme, with festivities often geared toward honoring veterans or supporting troops actively serving their nation.

Long-time NBC sports commentator Bob Costas, in an interview with CNN Monday, said he believed the national anthem at sports games had become entwined with pro-military, patriotic fervor in recent years.

"Sports and patriotism and the flag have been conflated to such an extent, that people can’t separate out any nuance," Costas remarked.

"If you go to see Hamilton, which is about the founding of the country, no one says, 'Wait a minute! Don’t raise the current until we play the national anthem'!"

Beginning in 2009 the US Dept. of Defense paid several franchises hundreds of thousands of dollars to produce those patriotic pregame displays, something that came to light years after the practice began.

Many fans have come to associate the national anthem with the concept of supporting the military, but Costas asserted the anthem and the flag aren't the exclusive domain of the armed services.

"We all respect their sacrifice. We all honor their sacrifice. And yet what it has come to mean is that the flag is primarily and only about the military," Costas told the CNN audience.

"This is no disrespect to the military. It's a huge part of the narrative. But Mart Luther King was a patriot. Susan B. Anthony was a patriot. Dissidents are patriots. Schools teachers and social works are patriots."

Sports and politics have been intersected and overlapped throughout history. For example, the US men's hockey victory over the Soviet the 1980 Olympics, forever known as "The Miracle on Ice," had added geopolitical significance coming in the midst of the Cold War.

And there was Jesse Owens's spectacular performance, winning four gold medals in the 1936 Berlin Summer Olympics, and in the process showing up Adolph Hitler on his home turf.

The 1968 Olympics were marked by the iconic Black Power salute by American athletes Tommie Smith and John Carlos on the medal stand, to call attention to the plight of African Americans still living in poverty and facing discrimination.

In fact, when then-San Francisco 49ers quarterback Colin Kaepernick decided to take a knee during the national anthem during the 2016 season it was to call attention to the disproportionate number of black men shot by police.

Costas said many of those debating the actions of Kaepernick and his fellow athletes have tried to make the debate about patriotism or supporting the military.

"To give him the extreme benefit of the doubt you’d say President Trump is insensitive to the racial implications to make comments like this at a rally in Alabama," Costas said.

Players have the same constitutional rights as any American to sit out the national anthem, thanks to a landmark 1943 Supreme Court decision in a case brought by two members of the Jehovah's Witnesses religious group.

"There were state laws in place that required students to stand for the pledge of allegiance and recite the pledge of allegiance, and members of the Jehovah’s Witnesses because of their religious beliefs did not feel they could do that," Hamline University political science professor David Schultz explained.

In West Virginia State Board of Education v. Barnette, the High Court held that the Bill of Rights protects citizens who want to be exempted from reciting the Pledge of Allegiance to the US flag.

"They said no government official has the right to prescribe orthodoxy, to tell us what we can believe," Schultz said.

"And that case applies to the NFL today, that the government, whether it’s Donald Trump or the local dog catcher, can’t tell people, the NFL, or any player what they can do, in terms of kneeling, standing or not showing up for The Star-Spangled Banner."