Here's what we know regarding events that began in southwestern Russia on Friday evening.

Elements in Ukraine aligned with the Wagner group, a mercenary outfit that had been engaged in some of the fiercest urban combat of the Ukraine war over the last year, claimed on social-media channels that their positions had been deliberately targeted by Russian ordnance.

Wagner group leader Yevgeny Prigozhin's ties with the Russian high command had been strained to the breaking point even before this reported attack. The mercenary leader had long complained that the Russian defense ministry — Chief of Staff Valery Gerasimov and Minister of Defense Sergei Shoigu by name — had bungled the war in Ukraine, covered up consequences of their errors, and hung Wagner, in particular, out to dry by failing to properly provide for its operations. For Prigozhin, the strike on Wagner’s positions was the last straw. In a variety of audio messages published on Friday night and early Saturday morning, he declared a "March of Justice," directing his forces to ride to Moscow. "If anyone gets in our way," Prigozhin warned, "we will destroy everything!"

Moscow took the threat seriously. A variety of Russian legal institutions issued orders for Prigozhin's arrest on the grounds that he had called for treason against the state. Officials within the Russian command structure who retained good relations with Wagner mercenaries were trotted out on camera, urging that cooler heads must prevail. Russian state television broke into regular programming in the early hours on Saturday with news anchors disputing the validity of Prigozhin's claims.

Unconfirmed footage of Russian forces erecting defensive preparations and setting up roadblocks began to bubble up from cities like Voronezh, Rostov-on-Don, and Moscow. More unconfirmed footage followed of Wagner columns rolling into the city of Rostov, where Prigozhin himself eventually appeared as his loyalists appeared to capture key military installations in and around the city. In a taped message, the Wagner group leader repeated his demand for Gerasimov and Shoigu's careers, pledged that his actions would not disrupt Russian operations in Ukraine, and promised to continue his drive on Moscow.

For his part, Vladimir Putin does not appear to appreciate his former "chef's" methods of helping Russia's war effort. Putin appeared on camera on Saturday morning in a five-minute address to the nation in which he called Prigozhin's insurrection "a stab in the back of our country and our people" and promised "decisive actions" to neutralize Wagner’s “armed rebellion.”

That's what we know for sure. What we only kind of know at this stage of these extraordinary developments is that the Wagner forces that entered Rostov met almost no resistance. There are reports that Wagner soldiers are advancing north along the M-4 highway toward Voronezh and Moscow. That is not to say that there has been no resistance to Wagner's advance. Some reports of fighting breaking out between Russian and Wagner forces have found their way into Western media outlets, and Prigozhin claims that Wagner group soldiers engaged and disabled three Russian helicopters.

Given the profound implications associated with the disturbing revelation that Russian mercenaries and Russian soldiers are engaged in combat on Russian streets, there's little more that can be said of this fraught development at this stage. But both sides of this emerging conflict have declared their intention to engage with one other, but we have not yet seen anything to suggest widespread conflict has broken out inside Russia. Yet.

And now, what we can only speculate upon as judiciously as possible:

Prigozhin insists that his rebellion is designed only to aid Russia’s war in Ukraine. But sacking the city of Rostov – a key strategic and logistical hub located on the other side of the border from the occupied Ukrainian city of Mariupol – will only complicate the Russian effort and frustrate one of its most important supply lines into southern Ukraine. The vast majority of Russian armed forces are still inside Ukraine, so the reserves Moscow can mobilize to frustrate Wagner's advance are likely limited to conscripts and the Russian equivalent of national guards. And the politically delicate nature of this assault on the legitimacy of the current Russian regime ensures that Putin cannot scorch the earth, even if he had the capabilities at the ready for such a disruptive operation. The Russian war in Ukraine has come home, and that is a politically dangerous proposition for Putin.

By way of metaphors, think of Wagner as something like Blackwater private contractors, but far more cultish and ideologically nationalist. Prigozhin claims they are 25,000 strong, armed with mechanized elements, tanks, and other heavy weapons. Their ranks are made up of soldiers of fortune, veterans of Russian wars, and hardened prisoners who were promised amnesty if they fought inside Ukraine. There, Wagner mercenaries caught some of the worst fighting of the war – months of brutal, street-by-street combat where they were poorly supported and undersupplied by their Russian sponsors. They are battle-hardened. They're angry. And they just captured a city equivalent to the size of Jacksonville, Florida – if Jacksonville, Florida, was also one of America's most well-stocked hubs of arms and ammunition.

Would you want to go to war with these people on your soil with limited expectations for immediate success and with the near-certain risk that such actions would cement the impression in the minds of your war-weary citizenry that the "special military operation" in the near abroad had evolved into a civil war at home? Probably not. But that is the choice Putin is confronted with now.

For Russia's critics in the West, this might be little more than an opportunity to sit back, soak in the schadenfreude, and observe as Ukrainian forces engaged in the counteroffensive exploit the chaos behind the Russian lines. Unfortunately, with the consideration of Russia's thousands of nuclear weapons in mind, the West's fortunes are tethered to Russian stability. Ambiguity about who is in control in the Kremlin – or, for that matter, Rostov, with all its hardware and nuclear infrastructure – isn't something Western capitals will take lightly.

Vladimir Putin is facing the most serious challenge to his legitimacy in his long presidency. He has every reason to fear other restive power centers within the Russian Federation will take advantage of his weakened state. A display of force, to the extent it can muster one, may strengthen Putin's political position, but it would also risk fomenting further instability and spooking Russian citizens. But failing to stop the Wagner advance could put the Putin regime itself in jeopardy.

Russia is becoming unglued. The next 48–72 hours are critical. They will determine the course of Russian and, therefore, European history for years to come.