American Exceptionalism – the doctrine that international norms do not apply to the United States – cannot go unchallenged. Yet President Trump clearly lives by it. It lies behind his decision to authorise the killing, by drone strike, of General Qassem Soleimani, who was the leader of the elite Quds Force of Iran’s Islamic Revolutionary Guards and one of the most powerful and dangerous men in the Middle East.

It looks 99 per cent certain that this was an illegal extra-judicial assassination. If there was clear evidence that Soleimani was about to order an imminent attack on American lives or property, then the principle of self-defence could apply. That must have been the view taken by the American military in undertaking the attack, as United States service personnel – as elsewhere – have a legal duty not to obey an order they know to be unlawful. American Exceptionalism does not override it. “Imminent” is the key word. It has to be demonstrated, not just asserted.