What’s happening in Russia? Here’s what we know.

A powerful mercenary military leader brought in by President Vladimir V. Putin of Russia to help salvage his floundering invasion of Ukraine turned his guns on the Russian Army this weekend, and his tanks prowled the streets of his own country.

What just happened?

With Mr. Putin’s regular forces bogged down in what a Russian general once predicted would be a “walk in the park” invasion, the president last year turned for help to a longtime political ally, Yevgeny V. Prigozhin, head of a notoriously brutal private army called Wagner.

Wagner did, in fact, notch wins in Ukraine — but it soon became clear that Mr. Prigozhin, a tycoon with an ego to match the Russian president’s, was not content to limit his fight to one front. Mr. Prigozhin bitterly lashed out against Russian military leaders, accusing them of incompetence and of undermining his fighters.

Then words turned to action.

On Saturday, Wagner forces seized control of key military facilities in the southwestern city of Rostov-on-Don and threatened to enter Moscow. Later in the day, Mr. Prigozhin appeared to back down, announcing that Wagner was “turning around our columns and returning to field camps according to plan.”

But for Mr. Putin, it is the most serious challenge to his hold on power since he became president in 1999.

The Russian president had held his tongue in recent months as his old ally grew increasingly outspoken, perhaps because Mr. Prigozhin was always carful to profess his personal loyalty.

That ended on Saturday.

“A stab in the back of our country and our people,” declared Mr. Putin to the Russian people, vowing “decisive actions.”

Here is what we know.

How big is the Wagner group?

Mr. Prigozhin’s shadowy private military company first emerged before Russia’s full-scale invasion of Ukraine, when the country seized the Crimean Peninsula in 2014. Since then, it has grown greatly in size and reach.

By some estimates, Wagner has perhaps 50,000 fighters at its disposal overall, though Mr. Prigozhin spoke this weekend of deploying a force half that size in his new battle against the Russian Defense Ministry.

Last year, when Wagner was called upon again to help in Ukraine, Mr. Prigozhin set out to expand his ranks. Freed from the constraints of the Russian bureaucracy, and insulated by his ties to Putin, he raised eyebrows by recruiting from Russian prisons.

Mr. Prigozhin offered inmates a deal: Fight for Wagner and earn your freedom. But the reward for many was not freedom but death. The soldiers were barely trained before being sent to the front lines, and those who tried to desert were executed.

Mr. Prigozhin, part of a charmed circle of Russian oligarchs with close ties to Mr. Putin, has long proved useful to him both at home and abroad. The Kremlin has used Wagner to exert its influence in Syria, Libya, the Central African Republic, Sudan, Mali and Mozambique.

Mr. Prigozhin was even implicated in a U.S. political scandal. In 2018, he was one of 13 Russians indicted by a federal grand jury on charges that they had interfered in the 2016 American election.

Is this a coup attempt?

No, said Mr. Prigozhin, even as his fighters seized Russian installations.

Yes, said Russian generals, as the Russian authorities opened an investigation into the Wagner leader for “organizing an armed rebellion.”

Mr. Prigozhin, who has repeatedly professed his loyalty to the Russian president, said his target was the Defense Ministry. “The evil borne by the country’s military leadership must be stopped,” he said in one of a series of voice recordings posted to the Telegram social network Friday night.

Whatever Mr. Prigozhin’s goal, his decision to deploy his forces in his own country had immediate reverberations.

On Saturday, Mr. Putin mobilized Russian troops to put down the assault. There were signs of active fighting along a major highway traveled by armored convoys, and regional governors urged residents to stay away from the corridor.

Mr. Putin said that military and civilian functions had “essentially been blocked” in an important military hub for the war in Ukraine, an implicit acknowledgment of some success by Mr. Prigozhin.

One Russian general urged Mr. Prigozhin’s fighters not to “play into the hands” of an enemy that he said was waiting for Russia’s internal political situation to worsen.

As the events played out in Russia, Ukraine’s armed forces posted three words on Twitter: “We are watching.”

What set off the fighting?

For months, when he has not been bragging about his force’s successes in Ukraine — including after a long and bloody battle in Bakhmut — Mr. Prigozhin has complained about military leaders.

He has accused Russian generals of failing to provide his forces with enough ammunition and of ignoring soldiers’ struggles. And he has been scathing about their prosecution of the war.

Then late Friday, Mr. Prigozhin took it to a new level, accusing the military of attacking his fighters’ encampments — a claim that could not be immediately verified. Calling his fighters “patriots of our motherland,” he pledged to retaliate.

Mr. Prigozhin also described the invasion of Ukraine as a “racket” perpetrated by a corrupt Russian elite.