Today, President Trump announced that the only way to stop Iran from getting nuclear weapons is to begin destroying the painstakingly negotiated agreement that is keeping them from getting nuclear weapons.

“History has shown that the longer we ignore a threat, the more dangerous that threat becomes,” Trump said, as though we had been ignoring Iran until now. “We will not continue down a path whose predictable conclusion is more violence, more terror and the very real threat of Iran’s nuclear breakout.”

So he’s going to withdraw his certification of their compliance, which means Congress now has to decide whether to reimpose sanctions. Congress will probably allow the deal to survive, with additional conditions. And Trump today said that, going forward, if he’s not satisfied, “the agreement will be terminated.”

And then what?

Or to put it another way: What exactly is Trump trying to accomplish? The answer may seem obvious, but it isn’t at all.

Presidents, we know, are supposed to have “vision,” a broad conception of where they want to lead the country. When they run, it’s often presented in vague terms; the closest Trump came as a candidate was promising that “We will have so much winning if I get elected that you may get bored with winning.” While in a sense “making America great again” was a kind of vision, presidents also need specific goals to guide their decision-making, a real conception of how they want things to turn out so that they can figure out the best way to get there.

Trump’s lack of those specific goals — or to put it another way, the lack of a defined end-state he’s trying to reach — may be one of his most underappreciated weaknesses as a president. Most people, even many in his own party, understand that he’s spectacularly uninformed about policy, not particularly bright and distressingly impulsive. But he also seems to have no idea where he’s trying to go, and we’re seeing it play out on multiple issues right now.

We’ll start with Iran. Ever since he was a candidate, Trump has complained that the nuclear agreement, which was negotiated not only between Iran and the United States but also with Russia, China, Britain, France, Germany and the European Union, is a terrible deal, while seldom getting specific about what exactly he objects to in its provisions. We knew what President Barack Obama was trying to accomplish with the deal in the first place: an Iran that, whatever else it might be up to, couldn’t threaten anyone with nuclear weapons.

What’s Trump’s vision? An Iran that not only doesn’t have nuclear weapons but also is a force for peace and stability, and maybe a liberal democracy to boot? Well, that would be great. How is pulling out of the nuclear agreement going to get us there?

Trump seems to believe that there’s some mythical “better deal” awaiting somewhere, and if he threatens to withdraw from the agreement, then the Iranian government will fall to its knees and say, “We submit! We’ll do whatever you want!” But of course it won’t, and the other partners aren’t interested in starting the process all over again either. If we do pull out, there’s a chance the agreement could collapse and Iran would resume its pursuit of nuclear weapons, which is exactly the thing the agreement is preventing.

It would be edifying to hear Trump or some of his aides and allies explain exactly how this scenario is supposed to play out and where it’s supposed to end up. But if they tried to do that, it would become obvious how little they’ve thought it through.

We see a similar vacuum of vision on other issues. Trump has decided to go whole-hog to destroy the individual health insurance market, with executive orders that will drive up premiums, send insurers from the market and potentially lead to many people losing their coverage. And what exactly is the health-care future Trump is aiming for with these actions? It’s almost impossible to tell. He often talks as if he’s a social democrat wanting government to provide for everyone (“We’re going to have insurance for everybody”), but then moves to remove government protections and move us toward a cruel Randian future more in line what what most Republicans would like to see. Can anyone say they have any idea what health-care system Trump envisions, and how it relates to the decisions he’s making now?

I suppose it’s possible that Trump actually believes that the actions he has taken will give millions of people fantastic, inexpensive insurance. If that’s true, then someone has taken him for a ride — and that illustrates how his ignorance makes him easy prey for ideologues who do have a vision. A president with a better grasp on policy would have at least have a sense of what course is likely to produce success and which outcomes are reasonable to predict. Trump, on the other hand, is apparently willing to believe any ridiculous story somebody tells him, if it ends with “Trump wins!”

A case in point: Conservative economist Kevin Hassett, the chair of the Council of Economic Advisers, apparently told Trump that just one of the White House’s proposed tax changes — allowing corporations to repatriate cash they have parked overseas and pay low taxes on it — would be such a spectacular shot of adrenaline to the economy that it would make every American family $4,000 richer. Sane economists, both Democrat and Republican, will tell you that this notion is utterly ludicrous. But it sounds good to Trump, so he has been touting the number as proof of how great his tax cuts are going to be.

That’s hardly the only fantastical idea he’s spreading around about tax cuts. Last month The Post reported that “Trump told a group of Democrats and Republicans on Tuesday that the tax framework could lead the economy to grow more than 6 percent a year, more than double what even his advisers had hoped for and a rate that many economists say is preposterous.” Does he actually believe that? Or has someone with an ideological agenda convinced him that it’ll happen, and he didn’t bother to think it through? You could ask the same thing about issues such as the North American Free Trade Agreement, where Trump has a simplistic impulse (trade agreements are bad!) but no real conception of exactly what he’d like to change and how doing so would produce positive results.

Presidents don’t need to be policy geniuses, but at the very least they need a sense of how cause leads to effect and a vision of what they’re trying to accomplish. That way they can tell whether what they’re doing is likely to take the country to the place they want to go. Trump has neither, which means he’s either being pushed around by people who have figured out how to manipulate him for their own ideological ends, or he’s flopping about aimlessly with no principles to guide him except if Obama did it, I should do the opposite. Either way, it’s not very encouraging.