One of the principal complaints against Donald Trump’s foreign policy is its incoherence. He quits a Pacific trade deal then ponders rejoining it. He abandons the Paris climate train before teasing about barging back on board. Steel tariffs for all! Well, unless you are exempted.

Trump has said it’s not chaos he’s presiding over but a conscious strategy. “We must as a nation be more unpredictable,” he said during the 2016 campaign. That way, he reasons everyone is kept on their toes. Vladimir Putin could read Barack Obama like a book. Not so him.

That’s one way to make sense of his actions, including his latest shocker – the decision to exit the nuclear deal negotiated with Iran by the permanent five of the UN Security Council plus Germany. He saw where the boundaries of accepted diplomacy lay and skipped right across them.

The other way is precisely to look back to the campaign. Trump is fixated on fulfilling promises made to his base. Some have proved beyond his reach. Like the wall. But where he can deliver he has, and the last of the very big ones was this: to kibosh the “insane” Iran deal that “never, ever should have been made”. Look at it that way and what he’s doing was entirely predictable.

As they say of cheating husbands, Trump has done it because he could. Almost certainly he did it also because the deal was something President Obama delivered. Undoing the Obama legacy is another of his main compass points.

Perhaps, also, he was emboldened by progress on North Korea. Trump the Reckless has been attracting Peace Prize talk. The two freshly installed, and entirely hawkish, members of his national security team, secretary of state Mike Pompeo and national security adviser John Bolton, surely nudged him forward. It was Bolton hovering by the door as Trump did his thing.

Iran parliament reacts after Trump's withdrawal from nuclear deal Show all 10 1 /10 Iran parliament reacts after Trump's withdrawal from nuclear deal Iran parliament reacts after Trump's withdrawal from nuclear deal Iranian MPs burnt a US flag in parliament after Donald Trump announced America's withdrawal from the nuclear deal. AFP/Islamic Consultative Assembly News Agency Iran parliament reacts after Trump's withdrawal from nuclear deal US President Donald Trump displays a presidential memorandum after announcing his intent to withdraw from the JCPOA Iran nuclear agreement in the Diplomatic Room at the White House. Reuters Iran parliament reacts after Trump's withdrawal from nuclear deal Some Iranian MPs reacted after Trumps decision by shouting 'death to America'. AFP/Islamic Consultative Assembly News Agency Iran parliament reacts after Trump's withdrawal from nuclear deal Iranian lawmakers prepare to burn two pieces of papers representing the US flag and the nuclear deal. AP Iran parliament reacts after Trump's withdrawal from nuclear deal Iran said it will hold talks with signatories to the nuclear deal after Trump's decision to withdraw from the accord, which it branded "psychological warfare". AFP/Islamic Consultative Assembly News Agency Iran parliament reacts after Trump's withdrawal from nuclear deal Several Iranian MPs stood chanting in the Tehran parliament. AFP/Islamic Consultative Assembly News Agency Iran parliament reacts after Trump's withdrawal from nuclear deal Iranian MPs burning a US flag AFP/Islamic Consultative Assembly News Agency Iran parliament reacts after Trump's withdrawal from nuclear deal President Hassan Rouhani addressed the nation in a televised speech in Tehran. He said he'd send his foreign minister to negotiate with countries remaining in the nuclear deal after Trump's decision to pull America from the deal, warning he otherwise would restart enriching uranium "in the next weeks." Iranian Presidency Office via AP Iran parliament reacts after Trump's withdrawal from nuclear deal Iran's press condemned Trump's withdrawal from a multi-party nuclear deal but was divided over whether Tehran should react with patience or withdraw itself. AFP Iran parliament reacts after Trump's withdrawal from nuclear deal Trump announced the US withdrawal from what he called the "defective" multinational nuclear deal with Iran, and said Washington would reinstate sanctions against the Islamic republic. AFP/Getty

“The United States no longer makes empty threats,” Mr Trump declared in the Diplomatic Room of the White House. There’s that promises thing again. “When I make promises I keep them.” As alleged proof, he revealed that Pompeo was within minutes of departing to Pyongyang for a second round of talks to set the terms of a summit between himself and Kim Jong-un.

Of course the biggest promise of all in 2016 was the one about putting “America first”. After signing his exit decree, he was forced by questions shouted by reporters to offer a variation on the theme. “This will make American safer!”

How so, exactly? No sooner had he finished speaking than the Pentagon was plunged into high alert about the possibility of imminent Iranian strikes against Israel, which was instrumental in pushing the president to his position. As never before, he has alienated European allies, notably France, Britain and Germany, which did all they could to pull him back from the brink.

So far – emphasis on that – so good on North Korea. But Tehran and Pyongyang are not the same. Trump has created a whole new nuclear crisis unnecessarily. He now faces two at once. And this is an additional crisis in a region already riven by military and diplomatic tensions. Hopes of stabilising Syria and Yemen seem suddenly further out of reach. Nor is it clear that this action won’t render the task of finalising a lasting deal with North Korea more difficult.

Trump, meanwhile, has sold this action on serial false premises. It is not just that he failed to spell out the enormous risks involved. It’s also that every utterance he has ever made on Iran has been laced with lies and mistruths. He said in his announcement that Iran is engaged right now in building a nuclear arsenal. Not true. Who is saying that? No one. There is no evidence.

He has repeatedly, and again in the Diplomatic Room, accused Obama of delivering $1.7bn to Iran the day the deal entered into force, as if it were some secret pay-off or bribe. In cash, in barrels, he said last week. There were never any barrels, a preposterous notion. He does it to stir up popular support for clobbering Tehran. Never does he acknowledge the truth that this was money owed to Iran for money paid to the US for military hardware that was never delivered.

And Trump again cited the case laid out by Benjamin Netanyahu, the prime minister of Israel, that Iran had spirited out a ton of secret documents that would have given the lie to its claim that it never pursued a nuclear military capacity. That too is a red herring, or rather a crock. Netanyahu, followed up by Trump, claimed this was evidence Iran was contravening the agreement. Not so said one nuclear proliferation expert after another. Nothing new there. Trump said that a better deal was there to be had in 2015. No there wasn’t, not by any stretch.

A new survey by Pew Research suggests that Americans, to whom Trump believes he is delivering on his promises, are not as dumb as he thinks they are. True, more Americans say they disapprove (40 per cent) than approve (32 per cent) of the agreement. But that doesn’t mean they think exiting is a sensible step. Fewer than half of those surveyed (42 per cent) said they are very or somewhat confident in Trump’s handling of the situation with Iran; 52 per cent say they are not.