Wow. Social media really is transforming the art of diplomacy. Donald Trump’s secretary of state, Mike Pompeo, has called the rulers of Iran a “mafia more than a government”. They are a bunch of “hypocritical holy men” who have devised “all kinds of crooked schemes to become some of the wealthiest men on Earth while their people suffer”. When Iran’s president, Hassan Rouhani, pleads for the mother of all peace instead of the “mother of all wars”, Trump retorts that if he ever says that again, he will see “consequences the likes of which few throughout history have ever suffered before … Be cautious!” Children are told that words can never hurt you, but there must be limits.

For Trump to deride Tehran as a mafia state while courting the leaders of Russia and North Korea is bit rich. Perhaps after his ham-fisted private diplomacy with Vladimir Putin and Kim Jong-un, he finds it easier to retreat to belligerence and abuse. More serious is the immediate bone of contention: the triggering this month of American sanctions following Washington’s withdrawal from the 2015 nuclear disarmament accord with Iran. Financial dealings with Tehran will have to freeze, followed in the autumn by a ban on imports of Iranian oil.

The resumption of sanctions is unlikely to destroy the regime or change its policies in the region. The US does not rule the world. But sanctions will almost certainly undermine moderation and reinforce conservatism within Iran. It will make an Iranian bomb more, not less, likely.

For US companies to be banned from Iran is one thing. For the world to be banned is another

More serious is what it will do for the cohesion of the west. No one else has reneged on the Iran deal. Yet Trump says that anyone who not only disagrees with America, but fails to conform to America’s sanctions, will in turn be sanctioned. Any company dealing with Iran from November will be denied access to American markets, as possibly might those dealing with them. For US companies to be banned from Iran is one thing. For the world to be banned is another.

That is a declaration by Washington of economic hostility towards allies more severe than the tariff war that Trump has already declared on Europe and China. It is more troubling in its implications than Trump’s antagonism towards Nato. It challenges other western countries in effect to side with China and Russia in continuing relations with Iran – and thereby implicitly jeopardising America’s strategic protection.

Trump is using social media to alter the norms of dealings between states. He is distorting language long designed to sooth tension and to reduce conflict between peoples by reducing it to bullying and contempt. The world is tempted not to take this too seriously, to regard Trump as a passing blip. But the risk is palpable. Salvaging the 2015 nuclear deal with Iran is an absolute priority for the otherwise disoriented leaders of the non-American west. They had better move fast.

• Simon Jenkins is a Guardian columnist