LANDOVER, Md. — It was undeniably a monumental Sunday for the National Football League. There were teams that were no-shows for the national anthem, and others that linked arms with owners. Players knelt and sat during the anthem, and some raised their fists.

But something was conspicuously missing from Sunday’s stage: a real discussion about the issues Colin Kaepernick wanted to highlight when he started the movement. He knelt during the anthem last season to protest social injustice and police brutality, and since then he has been a pariah no team wants to sign.

President Trump, in picking a fight with the league, reframed the issue as a lack of respect for the country and the flag, which may make it even harder for athletes to extend their one-day revolt into a political dialogue. On Monday, Mr. Trump again addressed the N.F.L. on Twitter, asserting a “tremendous backlash” against the league and its players for “disrespect of our country.” It was his 15th tweet in three days about athletes respecting the flag or the anthem.