In a further fracturing of the very shaky ceasefire between Iran and the United States, the Iranian Revolutionary Guard Corps (IRGC) this week shot down a US Army Apache attack helicopter near the Strait of Hormuz. At any other time, the US would have seen this as an unacceptable act of war requiring a very strong reaction. But where Donald Trump’s unpredictable social media output would usually have breathed hellfire and damnation, this time it talked a proportionate response.