Alternative histories are a fascinating business. Change a few key events, remove an assassination here or a diplomatic slight there, and the whole course of human civilisation can be transformed. On Saturday I was stood on Whitehall, just opposite Downing Street, pondering what might have been. Labour’s shadow Chancellor John McDonnell was addressing a protest against the “assassination” of Qasem Soleimani, the Iranian Revolutionary Guard commander who didn’t so much smear himself with blood as bathe in it. The killing, McDonnell insisted, must be unequivocally condemned by the British government.