Donald Trump’s arrival in the White House reflects a wider phenomenon: a new era of nationalism. He joins Vladimir Putin of Russia, Narendra Modiof India, Xi Jinping of China, Recep Tayyip Erdoğan of Turkey and a score of other nationalist leaders around the globe.

While it might be unfair to describe Theresa May as a nationalist, her announcement that she’s going for a hard Brexit reflects the pressure of English nationalism on the British right, and will encourage the nationalism of others. Of course, eras of nationalism are nothing new. But precisely because we have experienced them before, we know that they often start with high hopes and end in tears.

For now, the nationalists are giving one another the Trumpian thumbs-up across the seas. Paul Nuttall, the Ukip leader, says he is “massively excited” by the advent of President Trump, who in turn tells Michael Gove in the Times that he thinks Brexit “is going to end up being a great thing”. In a photograph that should become notorious, the Brexiteer Gove gives Trump a sycophantic thumbs-up, with a curiously goofy expression on his face, making him look like a teenage Star Trek fan who has caught 10 seconds with Patrick Stewart. The vice-president of France’s Front National responded to May’s Brexit speech by declaring: “French independence soon.” And so it goes on.

This world of mutually reinforcing nationalisms is also one in which both the relative power and the internal coherence of the west are being eroded from both sides of the Atlantic. The deterrent effect of the United States’ Nato security guarantee to Europe is being undermined from Washington itself. Meanwhile, we have had the amazing spectacle of the leaders of Russia, Turkey and Iran getting together to make a cynical deal over Syria. Erdoğan-supporting Turkish commentators revelled in the fact that neither the US nor Europe was even at the table….

[B]y their very nature, nationalisms are likely to clash sooner or later. Thus May’s insistence that Britain will leave Europe’s single market puts her on a collision course with Scottish nationalists, who have a referendum mandate for saying that Scotland wants to remain in the EU – and certainly in the single market. Moreover, 21st-century nationalisms exist in a high-pressure ecosystem of 24/7 media coverage and public scrutiny that would have appalled Bismarck, Disraeli and the tsar of Russia. Even authoritarian rulers such as Putin and Xi are riding the tiger.

By far the most serious of these potential clashes is that between China and the US. …