The UN General Assembly has always been as much a set piece for grandstanding nationalists as a forum for the advancement of shared goals. In that sense, this week’s high-level meetings, where heads of state from across the world have a chance to strut their stuff, will be little different. What makes them potentially explosive is the prospect of US president Donald Trump dominating the stage, both as host and as chairman of the Security Council, thanks to the revolving presidency.

By the time last year’s general assembly was held, Mr Trump had already set off on his own course. He had pulled out of the UN’s Paris accord on climate change. His Twitter sparring with North Korean leader Kim Jong Un was inflaming tensions on the Korean peninsula. But there was room for relief that he had only carried out some of the threats he had issued on the campaign trail. The Iran nuclear deal was still intact, and there was as yet no global trade war. There was even some residual hope that Mr Trump might settle into a more conventional foreign policy.

The US president did, however, give a strong indication at last year’s assembly of what was to come. “I will always put America first,” he told gathered world leaders. The effect since then of putting those words into practice has been to increase US isolation rather than to consolidate US influence. Even leaders among traditional allies, such as France and Canada, now ponder aloud whether Washington has gone rogue. America first is increasingly America alone. Short of a change of character, Mr Trump’s performance this week will reinforce that.