Months of crippling economic sanctions may be eroding Russia’s ability to wage war against Ukraine, with stockpiles of weapons, ammunition and heavy ordnance like the cruise missiles that hit Kyiv Monday running low. In an address scheduled for Tuesday, the director of a U.K. intelligence agency best known for cracking Nazi Germany’s Enigma encryption machine says he believes Russian President Vladimir Putin has committed “strategic errors in judgement” that have imposed a “staggering” cost in people and equipment. Diminished capabilities to wage war could shorten the conflict, now running for nearly eight months. “We know—and Russian commanders on the ground...