Russian President Vladimir Putin.

One night in mid-March, Alan Malcher, a British military veteran, dropped into the Queen's Arms, a working-class pub in north London. He took a seat at the bar and ordered his customary pint of Foster's. Within a few minutes, a stranger sidled up, ordered a drink and started a conversation. He soon brought up Russian President Vladimir Putin and began saying positive things about the Moscow-backed separatist civil war in Ukraine.

"He was going on about Putin being a strong leader," Malcher recalls. "Somebody to admire." The stranger's comments, delivered with a thick Slavic accent, made Malcher's security antennae vibrate: He had recently joined a Washington, D.C.–based think tank involved in combatting Russia's stealthy infiltration of American social media. So when the stranger made passing reference to Malcher's army service, he felt a twinge of apprehension. "There's no way he could have known that except via LinkedIn," Malcher says, referencing the professional online networking site where he and other critics of Moscow had been active in international affairs discussion groups. An expert in information warfare, Malcher reasoned that the Kremlin had dispatched the stranger to the Queen's Arms with a message: We know everything about you. Watch your step.