New Delhi, July 14 -- PARIS - When Polish railway workers found unexplained code embedded in their network's control systems last November, the disruption looked, at first, like a technical fault. By December, investigators were tracing the same signatures to a broader plot allegedly aimed at knocking out Poland's national energy grid. On Monday, France, the European Union and the United Kingdom put names and sanctions designations to a campaign those governments say stretched for years across more than a dozen European countries.

France announced it would summon Russia's ambassador to Paris over what Foreign Minister Jean-Noel Barrot described as "sabotage and espionage in a dozen European countries," a campaign he attributed to Russia'...