There is nothing surprising about Donald Trump’s admiration for Vladimir Putin. The would-be US president and the Russian leader share an authoritarian bent. They disdain multilateral engagement in favour of the raw politics of power. Above all, they are transactional. Deals are to be shaped by narrow definitions of national interest, unconstrained by international rules or shared values.

Mr Putin wants to erase the humiliation of the collapse of the Soviet Union. Mr Trump promises to “make America great again”. The reason for the Russian leader’s bad personal relationship with Barack Obama is the US president’s wounding refusal to indulge the fantasy of superpower parity. Perhaps Mr Trump has the better understanding of Russian psychology. He never ceases to praise Mr Putin as a strong and decisive leader.

The Republican party’s contender for the White House is not alone in cosying up to the Kremlin. Populists across Europe — Marine Le Pen’s National Front in France, Nigel Farage’s UK Independence party and the fascist Jobbik and Golden Dawn in Hungary and Greece respectively — have all tipped their hats to Moscow. Mr Putin also has sympathisers on the left. Britain’s Labour leader Jeremy Corbyn is more comfortable denouncing US “imperialism” than challenging Russian revanchism.

Until recently the foreign policy establishment was quietly preparing for a Hillary Clinton presidency. Mr Trump’s candidacy was a nightmare it would surely wake up from on November 9. The mood has changed. As the polls have tightened, Republicans and Democrats have begun to imagine Mr Trump as commander-in-chief. A dark quip among US generals — that they would remove the circuit boards before handing over the so-called nuclear football — no longer seems quite so amusing.

The fears are that the “shy” Trump supporters may not be showing up in the polls, that antipathy to Mrs Clinton could see centrists stay at home, and that the resolve of white working-class voters to punish the elites could overwhelm Mr Obama’s winning coalition of educated whites, Hispanics and African Americans. Faced with compelling evidence of Mr Trump’s mendacity, misogyny and racism, too many people reply that “he doesn’t really mean all that stuff”.

The organising fact for the rest of the world is that the US is the only nation that matters just about everywhere. It is no longer the hyperpuissance of the 1990s and has lost the appetite to remake the world, but the capacity of a thin-skinned, shoot-from-the-hip president to wreak havoc is chilling. A lot of people in Washington are trying to persuade themselves that the checks and balances in the system would restrain him. Judging from my conversations this week, they are not succeeding in the task.

The obvious fear is that a temperamentally unstable president Trump would lash out in a crisis. Robert Gates, the Republican former US defence secretary, says simply that he is “unfit to be commander-in-chief”. Mr Trump’s reaction to the latest bomb outrage in New York fitted the pattern. The US had to “knock the hell out of them . . . do something serious over there” — “them” being indeterminate and “over there” being the Middle East.

The bigger danger lies in Mr Trump’s promise to withdraw — to tear up trade deals such as the North American Free Trade Agreement, throw up trade barriers against China, repudiate the Paris climate change agreement and the nuclear deal with Iran, and abdicate responsibility for the security of east Asia and Europe. Mr Trump’s policies are shot through with contradictions but one constant drumbeat is his belligerent isolationism. America will go it alone. Hyper-realism, some call it. Dangerous is a better word.

The present global order — the liberal, rules-based system established in 1945 and expanded after the end of the cold war — is under unprecedented strain. Globalisation is in retreat. At a conference in New York organised by the US branch of the Ditchley Foundation I heard a distinguished American elder statesman remark that he has never known a period when the world had been simultaneously buffeted by so many upheavals and crises.

The list is a familiar one. Mr Putin is trying to redraw borders in Europe, the Middle East is in flames, European unity is fracturing, jihadi terrorism is spreading, pluralism is challenged by authoritarianism, China is contesting the status quo in the South China Sea and its neighbours are rearming in response, populists are storming the citadels across advanced democracies.

To Mr Trump, the answer is American retreat. He wants to build walls. He questions the US security umbrella in the Pacific — maybe Japan and South Korea should get their own nuclear weapons? He undercuts the credibility of Nato’s defence of Europe — the US might stand by if Russian troops marched into the Baltic states. There is no sense in any of this that American national security is safeguarded by alliances and international order.

If the polls are to be believed, Mr Trump has wrested momentum from Mrs Clinton in the presidential race. This does not mean he will win on November 8. The structure of the electoral college gives him only a narrow path to the White House. And there are three debates ahead. But the unthinkable has become the plausible. We should be more than worried. Neither America nor the world can afford a lurch into Trumpian isolationism.

philip.stephens@ft.com

Letter in response to this article

No, it is America that safeguards world security / Nathan Rollins