James Comey, the former FBI director, said of Donald Trump that he bore more resemblance to a mob boss than a democratic politician. In Mr Comey’s experience Mr Trump was a leader who demanded loyalty oaths, wanted absolute control, traded in lies and insisted on an us-versus-them worldview. Unfortunately these traits will undoubtedly be on display when the US president arrives at the North Atlantic Treaty Organisation summit in Brussels tomorrow.

Mr Trump’s tweets reveal a leader determined to turn the alliance into a protection racket. The president wants to shake down his military partners for some cash. He wants the summit to be all about who does and doesn’t spend 2% of GDP on defence. If partners refuse to pay? Well, the threat is left hanging: there are trade wars Mr Trump could fire up, or he could revive his campaign pledge to drop Washington’s commitment to automatically defending Nato allies if they are attacked. As Mr Comey said, it is like dealing with the mafia.

It is these kinds of actions that make Mr Trump so dangerous. The Nato alliance has helped mould the modern world and ushered in a democratic, liberal world order characterised by open trade and open societies, which after the collapse of the Soviet Union needed only to be lightly defended. This in turn contributed greatly to American peace and prosperity. A perception that the burdens are unequally shared is not new. It is almost as old as the alliance itself. Barack Obama worried that other nations did not act for themselves, waiting instead for Washington to lead. Mr Obama could be criticised for sermonising, but he was being straight. He saw that the defence of the multilateral rules-based order against jihadist terror, Russian adventurism and Chinese intimidation was essential. But it required other nations to shoulder their responsibilities. This seems fair. By contrast Mr Trump wrongly sees the US being disadvantaged, not advantaged, by the current global order. His tactics are to tear down this order rather than build it up.

In Brussels Mr Trump will hold a single bilateral meeting, with Jens Stoltenberg. The Nato secretary general should explain to the president how the US benefits from Nato. Not only do allies cough up for US bases; thousands of Nato troops are deployed in global hotspots. Where the 29-nation alliance has been slow is in modifying its stance to Russia, which has employed a highly effective mixture of deniable operations, disinformation and cyber-attacks to wrongfoot democratic opponents. These asymmetric measures are below the level of conflict. But Nato needs a collective defence provision when dealing with them.

Nato’s top brass will have their work cut out. The US president prefers the argument of power to the power of argument. He thinks little of humiliating allies over the Iran nuclear deal or snubbing them over the Paris climate change pact or infuriating them with tariffs. This could lead to anarchy, as nations conclude that they are freer to act with impunity. With rising powers such as China, it might end in confrontation. The US president’s description of Nato as being “as bad as Nafta” at the last G7 summit is not a good sign. The twin pillars of Europe’s place in the world are law-based global order and the transatlantic alliance. Mr Trump will find to his cost that Europeans cannot choose between them. Order cannot be imposed; it needs to be accepted. Mr Trump likes to think of politics as business, but he won’t strike good, lasting deals with allies – and enemies – without rules that everyone plays by.