On a single afternoon last autumn, engineers at a British power grid operator quietly rerouted traffic through backup systems while analysts at GCHQ's cyber arm traced the intrusion to a server cluster linked to a foreign intelligence service. That incident was one of more than 200 recorded in the twelve months to May — a figure that Richard Horne, chief executive of the National Cyber Security Centre (NCSC), disclosed this week in a speech at the Royal United Services Institute in London. Roughly three-quarters of those incidents, he said, are believed to have been carried out by actors working on behalf of states, with Russia, China and Iran identified as the principal sources of hostile activity. The targets were not abstract.

Critical national infrastructure — a category that spans nuclear facilities, electricity grids, hospitals and airports — formed the core of the attacks. Horne rejected the idea that this amounts to a neatly contained confrontation. "It is far more akin to a football or basketball game, played across a large field of play, where success depends on how you operate across the entire pitch," he told his audience, drawing a picture of a contest that reaches from corporate boardrooms to domestic sofas. The metaphor matters: it implies that no single defensive line can hold.

What makes the warning particularly urgent is the role that artificial intelligence is expected to play.