Just over five years ago, the head of the Russian armed forces, General Valery Gerasimov, gave a speech in the Kremlin about the future of warfare. You don’t need guns and tanks to defeat an enemy, he said: there are, nowadays, more effective ways of winning. Computer hacking, TV propaganda, criminal skullduggery – all can unnerve an opponent and weaken alliances. This became known as the Gerasimov doctrine, and its latest victims are still lying in Salisbury hospital. Not so much a declaration of war, but a reminder of one already underway.

This is what Theresa May has been trying to get across at the Brussels summit. She wants other European countries not to repeat her mistake. As Home Secretary,...