Just before noon on March 29, President Trump tweeted a blood-curdling and very specific threat: “If Mexico doesn’t immediately stop ALL illegal immigration coming into the United States throug [sic] our Southern Border, I will be CLOSING . . . the Border, or large sections of the Border, next week.” This was not, to be sure, the first time he had threatened to close the border. He had previously done so on Oct. 18, Nov. 24, Nov. 26 and Dec. 28 as part of his agitation against caravans of refugees and the general flow of undocumented immigrants.

But this was the first time he had ever attached a timeline to his threats. “We’ll keep it closed for a long time. I’m not playing games,” Trump told reporters on March 29, while complaining that “Mexico is making absolutely a fortune with the United States. They have a trade surplus of over $100 billion, which is far bigger than anybody understands.” (Mexico’s actual trade surplus with the United States, according to Trump’s own trade office, was $63.6 billion in 2017.) “It certainly isn’t a bluff,” White House counselor Kellyanne Conway assured Fox News two days later. “You can take the president seriously.”

Actually, it turns out, you can’t take the president seriously. He is playing games.

On Thursday, Trump backed off. Speaking with reporters, he claimed a phantom victory, praising Mexico for “doing a very good job in the last three or four days since we talked about closing the border,” even though, as The Post noted, “Mexican authorities have said they have not altered their enforcement policies.”

“We’re going to give them a one-year warning, and if the drugs don’t stop or largely stop, we’re going to put tariffs on Mexico and products, particularly cars,” Trump said. “And if that doesn’t stop the drugs, we close the border.”

So President Wormer is placing Mexico on “double secret probation”: If the Mexicans don’t stop all illegal immigration, then he will really truly close the border . . . in a year. Or so. In the meantime, he threatened to impose 25 percent tariffs on made-in-Mexico cars, even though this would violate the terms of the newly negotiated U.S.-Mexico-Canada Agreement on trade.

There is, of course, no reason to assume Trump will be any more willing to carry out his threat in the future than in the recent past. Closing the border would have a “potentially catastrophic economic impact,” as Senate Majority Leader Mitch McConnell (R-Ky.) noted. Doing so would interrupt $615 billion in two-way trade and could throw 5 million Americans out of work. Automobile production would grind to a halt. Avocados would disappear from store shelves. No guacamole? Now that’s a national emergency.

And no matter how much economic damage Trump inflicts, Mexico cannot fully control its borders any more than the United States can.

The good news: Trump did not carry out an ill-advised ultimatum. The bad news: The president of the United States has once again been exposed as a very bad poker player. Trump is the maestro of empty threats. The pontiff of broken promises. The bard of bluster, bluff and BS. You cannot take him seriously — and increasingly, few do, either at home or abroad. Both foreign governments and savvy Americans by now know to discount most of his rhetorical effusions.

Indeed, at the very time that Trump was retreating from his threat to close the border, he was also backtracking from his promise, also made the week before, to produce a health-care bill. Now, he says, that will happen right after the 2020 election if voters simply hand back control of the House to Republicans. Vote Republican to find out what’s in the health bill! So, at some point in the future, the GOP will produce a bill with better coverage and lower costs than Obamacare. Yeah, right. Of course, in 2016, Trump vowed to unveil such legislation on his first day in office. More than 800 days into his presidency, a Republican health-care bill remains as chimerical as a good 5-cent cigar — or Trump’s tax returns.

These are only a few of the many promises and threats Trump has not delivered on. He has not locked up Hillary Clinton. He has not forced Mexico to pay for a border wall. He has not kept the government shut down until Democrats funded the wall. He has not rained “fire and fury” on North Korea. He has not torn up NAFTA. He has not pulled U.S. troops out of Syria. He has not loosened the libel laws.

It’s a good thing he has not implemented any of these harebrained ideas. Thank goodness his advisers talked him off the ledge. But with his empty verbiage, Trump is decreasing his own bargaining leverage with adversaries such as China, Iran, Venezuela and North Korea, while raising the risk of a fatal miscalculation in an international crisis. A president’s words can mean the difference between war and peace. Even if Trump means what he says, how would anyone know? Being a fabulist may be acceptable for a real estate developer. It’s destructive and self-defeating for a commander in chief.

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