Washington • Don’t tell me the issue is mental health. Save the nonsense about “good guys” with weapons somehow being the answer. The truth is this: There would have been no tragic shooting Wednesday in Parkland, Fla., if a troubled young man had not gotten his hands on a military-style assault rifle and as much ammunition as he wanted.

Many people knew that Nikolas Cruz was troubled, violent and liable to explode. Yet nothing kept him from acquiring the gun he allegedly used to kill 17 students and faculty members at Marjory Stoneman Douglas High School, in yet another senseless slaughter of the kind that still shocks but no longer surprises.

President Trump addressed the nation, saying we must “answer hate with love” and vowing to “tackle the difficult issue of mental health.” But he made no mention of gun control — meaning he might as well have said nothing at all.

According to news reports in The Washington Post and other outlets, Cruz, 19, had been repeatedly suspended and then expelled from the high school for disciplinary problems. He had sought and received treatment at a mental health clinic, but stopped attending. He showed signs of depression. His posts on social media were so unsettling that they scared acquaintances away. His father died several years ago and his mother, with whom he was close, passed away around Thanksgiving. According to The Associated Press, he had joined a white nationalist group.

And he had a fetish for guns.

To say there were warning signs about Cruz is a gross understatement. There were flashing red lights and blaring sirens.

“I think everyone in this school had it in the back of their mind that if anyone was supposed to do it, it was most likely going to be him,” a former classmate told the Post — “do it” meaning the kind of homicidal rampage we saw Wednesday. A math teacher at the school told the Miami Herald that Cruz had threatened students before being expelled, and that “he wasn’t allowed on campus with a backpack on him.”

It was obvious that a tragedy might be coming. Yet, as a society, we refused to take the steps that could have prevented it.

How many alienated and disturbed young men are out there, rattling around peaceful suburbs like Parkland? Many thousands, surely. Some will get the help they need; some won’t. Most will never act on their violent urges; some will.

It is impossible to imagine a mental health system with the scope, authority and resources necessary to prevent every Parkland or Newtown or Columbine. In Cruz’s case, in fact, the existing system worked: He got professional help. School administrators knew he could pose a threat. According to news reports, a tipster even tried to alert the FBI that Cruz wanted to be “a professional school shooter.”

Yet in February 2017, Cruz was able to legally purchase an AR-15-style assault rifle from a local gun shop. With no criminal record or anything else in his past that would raise an official red flag, he sailed through a background check.

Apologists for the National Rifle Association will cynically use this circumstance to argue against a common-sense measure that four out of five Americans support: universal background checks for gun purchases. That wouldn’t have stopped the Parkland massacre, they will claim — diverting attention from the fact that universal checks could have prevented some mass shootings in the past and would surely prevent some in the future.

At the heart of the matter, though, lies the gun.

There are disturbed individuals like Cruz in countries around the world, but only the United States suffers mass shootings so regularly that they have almost become routine. Why? Because you can’t shoot up a school or a night club or a country music concert the way it was done in Parkland, Orlando and Las Vegas if you don’t have access to weapons of war that were designed for the battlefield.

The Supreme Court has made clear that the Second Amendment, while guaranteeing the right to keep and bear arms, is not absolute. The Bill of Rights was written in the era of muskets and dueling pistols. We don’t allow private citizens to own surface-to-air missiles capable of downing an airliner, so why do we let them own assault rifles designed not to shoot targets or game but to kill human beings in large numbers?

This is madness. Look at those grieving parents in Parkland. Know that because of the NRA, others surely will soon join their ranks.

Eugene Robinson