The only certainty about President Donald Trump’s visit next week is that it will unloose a fresh torrent of clichés about the “special relationship” between Britain and the United States. The man himself, if he feels benign, may toss us a bone by according it lip service.

It is certainly true, or has been through the past seven or eight decades, that the two nations share many interests and values which it has advantaged both to pursue in harness. But the besetting sin of some British people, including successive prime ministers, is to delude themselves that sentiment influences US behaviour.

Jacob Rees-Mogg wrote in The Times last month, outlining an almost euphoric vision of a post-Brexit Anglo-American concord. His remarks betrayed a heroic ignorance about…