In a dramatic turn of events, Russia’s new defense minister, Andrei Belousov, reached out to U.S. Defense Secretary Lloyd Austin with alarming news: Russia suspected a covert Ukrainian operation against them had U.S. backing. This unexpected call, which came after several years of limited communication, threw the Pentagon into a frenzy as officials scrambled to clarify the situation and prevent any potential escalation. Despite Ukraine's secretive military maneuvers causing tension, including strikes deep inside Russia, the real story is the fraught diplomacy behind the scenes as the U.S. and Russia navigate this high-stakes game of military and political chess.

Defense Secretary Lloyd J. Austin III received an odd request earlier this month from an unexpected caller: a request to speak with his Russian colleague.

Only five times since Russia invaded Ukraine in February 2022 had Mr. Austin had a phone conversation with Russia's military minister, nearly all at the Pentagon's...