LONDON, June 6 -- The number four, in British defence circles, has acquired a specific and unsettling weight. Four years, the timeline now repeatedly attached to the moment Russia could, according to western intelligence assessments, move against a NATO member state. On Friday, speaking to workers at Stark, a drone and autonomous-systems manufacturer in Swindon, Prime Minister Keir Starmer made the figure public in the starkest terms his government has used.

"It is our intelligence assessment, and the assessment of other countries in NATO, that there could be an attack by Russia on NATO as soon as 2030," Starmer told employees gathered inside a facility where drones already being deployed in Ukraine are produced. "So you can see the urge...