The Wagner mercenary group's march toward Moscow revived an old fear in Washington: What happens to Russia's nuclear stockpile in the event of internal turmoil?
Wagner's boss, Yevgeny Prigozhin, ended the rebellion against the regime after a deal brokered by the Belarusian leader.
That eased immediate concerns of a major conflict within Russia, but signaled that the power of Russian President Vladimir Putin is waning.
Images of tanks on the streets of Russian cities brought to mind the failed 1991 coup by communist hardliners, raising concerns about the security of the Soviet nuclear arsenal, former US intelligence officials say.
"The intelligence community is going to be super focused on the [Russian] nuclear stockpile," says Marc Polymeropoulos, a former senior CIA officer who oversaw the agency's covert operations in Europe and Eurasia.
"You want to know who's in control of the nuclear weapons because you're worried that terrorists or bad guys like [Chechen leader Ramzan] Kadyrov might come next," says Daniel Hoffman, a former senior CIA officer. -s, who served as the agency's station chief in Moscow.
Kadyrov sent thousands of his forces to Rostov-on-Don - the southern Russian city occupied and then abandoned by Prigozhin's fighters - vowing to help put down the revolt.
US officials say they see no immediate threat to the security of Russia's strategic and tactical weapons.
Kremlin spokesman Dmitry Peskov said the deal, which returned Wagner's fighters to their camps, was aimed at avoiding confrontation and bloodshed.
"We have not seen any changes in the deployment of Russian nuclear forces," said a spokesman for the US National Security Council.
"Russia has a special responsibility to maintain command and control of its nuclear forces and to ensure that no actions are taken that endanger strategic stability," he said.
But the security of these weapons is a constant concern for Washington.
US intelligence agencies said in their annual threat assessment for 2023 that "Russia's nuclear security remains a concern, despite improvements in material protection and control".
The nuclear chain of command
A worrisome scenario is the possibility that a military faction gains decision-making ability over some of the weapons, if the divisions over the war in Ukraine that Prigozhin's rebellion exposed flare up again.
The United States and its allies wonder how any new authority will use the weapons, Hoffman says.
"And he may not play by the same rules that Putin does," he says, noting how the Russian leader has not acted on the nuclear threats he has made in response to Western support for Ukraine's war against Russian invaders.
Russia's nuclear arsenal is the largest in the world. According to estimates by the Federation of American Scientists in 2022, Russia has 5,977 nuclear warheads, compared to the 5,428 the US is estimated to have.
Gathering information about the command structure of Russia's strategic forces and their security and other aspects has long been one of the highest priorities of US spy agencies, former CIA officers say.
This job was made more difficult by Putin's decision, in August 2022, to halt US inspections of Russia's nuclear sites, as provided for in the New START agreement.
That decision made Washington heavily dependent on spy satellites to assess the security of nuclear weapons sites and the movements of nuclear warheads, Polymeropoulos says.
"This has always been a super-high priority of the US — information on command and control of nuclear weapons in Russia," says Hoffman.
"We all know that they are dangerous, that's why we had all those treaties, where we had a lot of transparency, and now they have disappeared", he concludes./REL