For the first time in post-Soviet Russian history, Moscow has arrested a U.S. journalist on espionage charges — which his newspaper vehemently denies. What messages is the Kremlin sending with this revival of a relic of the Cold War? And what comes next?

A Moscow court on March 30 authorized the detention of U.S. journalist Evan Gershkovich of The Wall Street Journal for two months on suspicion of espionage. The Federal Security Service (FSB) claimed…