Donald Trump has taken a wild swing at his predecessor’s key foreign policy legacy, the milestone 2015 Iran nuclear accord. By refusing to “certify” Iran’s compliance, Mr Trump has set events on an unpredictable course. He had until 15 October to “certify” the deal’s implementation. He had done so twice already since coming to office. But this time, though nothing substantial has changed, he’s noisily refused. Does this matter? He cannot, alone, pull the US out of this deal but he has raised the spectre that this might happen “at any time”. He wants measures taken to counter Iran’s “destabilising actions”, and to “deny all paths to a nuclear weapon”, though without much clarity as to what this might entail. None of this contradicts the agreement formally, but it will all weaken it. Congress now has 60 days in which to decide whether to vote to reimpose sanctions whose lifting was a essential part of the quid pro quo contained in the agreement. Even if that happens, European allies who are party to the nuclear deal, along with Russia and China, have all clearly indicated they will act to preserve it.

The 2015 deal offered the best possible assurances that Iran’s nuclear military activities would be contained for roughly 10 years. It imposed strict international inspections, and provided strong incentives through sanctions relief. By defusing the nuclear crisis, it helped consolidate the more pragmatic or moderate factions within Iran’s power structures. It has deprived Israel’s leader of a pretext to threaten Iran with military strikes. It has helped to prevent the arms race in the Middle East from taking on entirely new proportions.

Now, all these positive outcomes may start to unravel. Iran has promised a “crushing” response if its Revolutionary Guards are targeted by US sanctions – something Mr Trump has now called for. His provocation could lead to Iran pulling out of the nuclear accord – which may well be his tactical goal.

The emerging diplomatic picture is unprecedented: a US president finds himself at odds not only with his own close advisers, but with all the key international players involved in a major crisis over international security – including close European allies. All still support the nuclear deal with Iran. “America First” has become “America alone” at the moment when it is clear that America, alone, has far less power than it still supposes.

Mr Trump’s decision is the culmination of years of rhetoric lambasting the Iran deal as “the worst ever”. He has been encouraged by Benjamin Netanyahu, the Israeli prime minister. No one else believes that Iran has violated the terms of the deal. Even Mr Trump now claims only that Iran has violated the “spirit” of the accord. The regular assessments provided by the International Atomic Energy Agency, the UN-related body missioned with monitoring Iran’s activities, have found no violations. Iran’s behaviour in the region may well be criticised, but the 2015 accord had nothing to do with the actions of Iranian-controlled armed groups in Syria, Iraq or Yemen, nor with its support to Hamas and Hezbollah. The agreement was solely focused on curtailing the regime’s uranium enrichment programme and removing its ability to build a nuclear weapon within a relatively short period if it took the decision to do so.

There is no need to panic. Just as the agreement has its limits, so does Mr Trump’s angry rhetoric demonstrate his. Whether or not he has recognised reality himself, or been led carefully round it by his handlers, he has not quite taken any action which Iran would find completely unacceptable. He threatens and blusters, but the agreement will stand unless Congress decides to break it. This is where all the pettiness of his character works for the good. Because he is interested only in victory, he may fail entirely to notice that he has backed down from some of his own rhetoric. But Congress must now refuse to impose sanctions that would definitively break the deal.