Unlike pre-emptive wars, preventive ones are deemed unlawful because they grant the powerful licence to strike at will. The distinction is important; it is why many European governments rejected Russia’s justification of its invasion of Ukraine by claiming to head off a future threat. Law cannot be optional for allies and binding only for adversaries. The domestic foundations of Mr Trump’s action are also shaky. There’s little public support in the US for this attack, and Congress was not asked to authorise hostilities. There will be even less appetite as the civilian death toll mounts and US soldiers come home in bodybags.