Britons are never happier than when ridiculing the vulgarity of American politics. Donald Trump’s acceptance speech at the Republican convention in Cleveland last night was therefore a gift. It was as vacuous a catalogue of cliches as Barack Obama’s “Yes we can” speeches in 2008.

This is colouring-book oratory, and intended as such. A more serious question is, what would a Trump presidency be like for the outside world?

Trump’s foreign policy line has been clearer than his domestic one: it is a revival of Republican isolationism. He attacks Hillary Clinton for bringing “death, destruction and weakness” to the Middle East, citing the interventions in Iraq, Libya and Syria.

He refrained from mentioning the role of his Republican forebear George W Bush. But he has indicated a clear rejection of world free trade, immigration, and any notion of American sanctuary. Trump’s hostility to Mexican migrants and incoming Muslims has been only mildly diluted.

On collective security, he has attacked “our allies riding on our back”, in a stark invitation to Europe to start thinking afresh about its defence priorities.

Trump is right to point out that Obama’s global antics have been ham-fisted. The continuing continuing chaos in the Middle East may be an easy target, but he has persistently opposed these interventions. Obama’s fascination with the drone as a weapon of aggression, his failed “reset” with Russia, the decline in relations with China, and the clumsy remarks about Brexit all illustrated an ineptitude as self-appointed global policeman.

Last week American jets massacred 73 civilians, including 50 women and children, in the Syrian village of Manbij. Imagine if Isis had done this. Obama’s wars remain unresolved and immoral.

A jolt of realpolitik from an isolationist Republican would be no bad thing. Of course, Bush too was vigorously isolationist in his pre-9/11 mode in 2000, but the days when the world’s collective security hung on a Washington heartbeat are over. Behind the bombast, a period of transatlantic withdrawal and reflection is in order.

As far as Britain is concerned, Trump welcomed Brexit, albeit as a token of his own popular defiance against a ruling class. He would presumably honour this by reversing Obama’s end-of-the-queue attitude to a trade deal with Britain. Either way, the tectonic plates are shifting. A Trump presidency would, like Brexit, be a leap into the unknown. But even the darkest clouds can have silver linings.