But Washington’s cowardly lawmakers have only made it easier by repeatedly refusing to pass even the most obvious and least burdensome measures, like universal background checks and a strong assault weapons ban, as well as not empowering the F.B.I. to block firearms sales to people on a terror watch list. Mr. Mateen — whom the F.B.I. investigated in 2013 and 2014 for suspected terrorist ties — may well have been stopped from purchasing his guns if such laws had been in place. And he was not alone in his choice of weapon. Before the slaughter in Orlando, 95 percent of the deaths resulting from domestic terrorist attacks since Sept. 11 were inflicted by guns, according to a 2015 analysis. Radical Islamic terrorist groups openly advertise America’s lax gun laws to encourage those who want to carry out attacks here.

This is not complicated. Hillary Clinton, the presumptive Democratic nominee for president, gets it. In a speech on Monday afternoon, she said, “If the F.B.I. is watching you for a suspected terrorist link, you shouldn’t be able to just go buy a gun with no questions asked.”

Common-sense measures like these do not infringe on the Second Amendment, however much the National Rifle Association likes to pretend that the words “well regulated” have somehow disappeared from the amendment’s opening clause. Without history or logic on its side, the gun industry and its supporters instead continue to hawk only one solution: more guns, and more “good guys” carrying them, prepared to kill someone if they feel they need to. As President Obama said on Sunday, “We have to decide if that’s the kind of country we want to be.” The answer should be no.

In any case, the events of early Sunday morning once again demolished that particular Wild West fantasy. The nightclub was, in fact, protected by a good guy with a gun — an off-duty police officer, who was unable to stop the carnage.

No law will stop all gun crime, but smart laws will reduce the chance that the people at greatest risk for committing violence with guns will be able to get their hands on the deadliest weapons.

America’s gun-violence epidemic is a national public health crisis, and if it were treated like every other public health crisis, it would be studied in depth and attacked with evidence-based approaches. But Republicans in Congress have blocked that path, too, repeatedly barring federal funding into research on gun violence, because they are afraid of what it will show.