The Wagner mercenary group's march on Moscow has revived an old fear in Washington: What happens to Russia's nuclear stockpile in the event of domestic upheaval?

An agreement on Saturday by Wagner's boss, Yevgeny Prigozhin, to order his fighters back to their camps quelled immediate worries of major conflict inside Russia. But the episode signaled that Russian President Vladimir Putin's grasp on power is weakening.

Images of tanks on Russian streets brought to mind the failed 1991 coup by communist hardliners that raised concerns about the security of the Soviet nuclear arsenal and the possibility of a rogue commander stealing a warhead, said former U.S. intelligence officials.