First, a bomb exploded on the roadside.
Then, machine-gunners opened fire on the immobilized convoy.
When the ambush was over, five bodyguards, a press secretary and Aleksei B. Mozgovoi, the rebel leader, lay dead.
Rebels have been fighting in Ukraine for more than a year now, but the bloody assault this May was different: a massacre carried out by separatist rebels against another rebel group and former ally, the famed Cossacks, in a clash over the groups’ competing territorial claims.
The bitter feuding between the groups raises the prospect of greater factional fighting among the rebel forces, deepening an already grave humanitarian situation and possibly complicating matters for President Vladimir V. Putin of Russia, whom the West accuses of arming and supporting the separatists.
“The Cossacks are facing reprisals” from the separatists, Lyubov A. Korsakova, the editor of a Cossack newspaper, the Front Bulletin, said in an interview here in the traditional Cossack capital.
“They started to disarm the Cossacks, and not only to disarm them, but to kill them.”
When the rebellion erupted in eastern Ukraine last year, the Cossacks, the whip-wielding, onetime horsemen of the southern Russian steppes, sent hundreds of young men as volunteers to fight alongside the rebels.
Renowned warriors, as well as darlings of the Putin-era Kremlin, they lent a steely organization to the often ragtag separatist forces.
As the fighting died down, the Cossacks established at least de facto control over three eastern Ukrainian towns which they claimed as “Cossack Republics” and subjected to harsh, traditional punishments, like public horsewhippings for petty criminals.
At the peak of their success last year, the republics run by Mozgovoi and two other Cossack commanders, or atamans, Nikolai I. Kozitsyn and Pavel L. Dryomov, claimed to control 80 percent of the Luhansk region, including major towns, strategic roads and border crossings to Russia.
They were closer than ever before to realizing a long-held dream of having an independent Cossack state.
However, their former separatist allies were not about to cede hard-won territory, and began to purge the Cossacks from their ranks.
At least dozens of the fighters sent to Ukraine by the Don Cossacks, the main Cossack group, have died in mysterious ambushes in recent months, according to local news reports.
The Ukrainian Interior Ministry, which tracks the infighting, estimates a death toll for the Don Cossacks as high as 200, according to Valentyn V. Tkalych, a spokesman.
Ms. Korsakova, who has been a vocal advocate of the Cossacks within the pro-Russian side in this internecine conflict, put the toll at more than 100.
The fighting is alarming Cossack organizations, which say Cossack fighters are now being expelled from one of the separatist enclaves they helped create, the Luhansk People’s Republic.
Kozitsyn, the most senior Cossack commander, left eastern Ukraine on May 14, telling his supporters he did so to avoid bloodshed.
Outside Russia, the Cossacks tend to be viewed as cartoonish anachronisms, with their whips, papakha fur hats and horses.
But in southern Russia, they are a serious political force.
Under the czars, the Don Cossacks ruled as Russian vassals over the Wild Field, an area encompassing parts of southern Russia and eastern Ukraine.
They were an anti-Bolshevik force in the civil war, suffered repression under Stalin, and then fought as cavalry on both sides, with the Nazis and the Soviets, during World War II.
In 1962, the Soviet authorities put down a Cossack uprising in Novocherkassk, killing 26 people.
After the breakup of the Soviet Union, Kozitsyn emerged as the best-known advocate of the restoration of Cossack autonomy over the Wild Field, mirroring the demands of other Russian minorities for independence.
The Don Cossack independence movement, though, fizzled without a fight.
But it never disappeared.
As Putin has emphasized traditional values, the Cossacks, known for their bravery, pre-modern codes of honor and Orthodox faith, were granted increasing roles in law enforcement in southern Russia.
Those joining this Kremlin-sanctioned Cossack revival were called “registered” Cossacks.
Those who resisted what they saw as co-optation became known as “free” Cossacks.
“Those who fought on the side of these republics had a choice: Either join the regular army, or leave,” said Ivan P. Konovalov, the director of the Center for Strategic Trends Studies, a research organization in Moscow.
“The free Cossacks were always a danger.”
When the war in Ukraine began a year ago, the free Cossacks nonetheless proved useful to the Kremlin.
They crossed the border and fought in militias loosely organized under the Great Don Cossack Army, led by Kozitsyn.
Korsakova said that the collapse of the border last year had rekindled the earlier dreams of autonomy.
But not for long.
Korsakova, 51, spoke from her hospital bed here, where she is recovering from an attack by unknown assailants with metal pipes outside her apartment in June.
She said the attack was a reprisal for her reporting on the Cossacks’ predicament in Ukraine.
The police declined to open an investigation, saying she had slipped and fallen.
The attackers broke her shinbones and left her unconscious on the sidewalk.
They also struck her in the mouth, a gesture, she said, indicating that she should stop talking about the infighting.
It is not known who was responsible for the attack, but her reporting contradicted the position of the Moscow authorities that the pro-Russian separatists are united.
“A purge of Kozitsyn’s people is underway,” Korsakova said.
“People who fought with Kozitsyn are disappearing. Only fools cannot see it.”
After a series of attacks on their leaders, many of them fatal, the Cossacks went into open revolt.
“Is this why we intervened? Is this why we died?”
Dryomov, the Cossack chieftain of the town of Stakhanov, said in a video posted online in January, addressed to Putin.
“This is our land. We were born here, we live here and, God willing, we will die here.”
He went on to ask Putin to remove Igor Plotnitsky, the head of the Luhansk republic.
In response, Plotnitsky set an April 4 deadline for the Cossacks to integrate into the Luhansk army.
After his murder in May, a spokesman for Ukraine’s Interior Ministry, Anton Herashchenko, said intelligence suggested operatives from Russia’s Main Intelligence Directorate had struck his convoy.
The Luhansk republic blamed Ukrainian special forces.
In May, the remaining Don Cossack battalions in eastern Ukraine, including Dryomov’s unit, were renamed “territorial defense brigades” and integrated into the Luhansk People’s Republic army.
Today, even the highest-profile Cossack to have fought in eastern Ukraine, a burly, bearded fighter who goes by the nickname Babai and was one of the first Russian soldiers to fight in the Ukrainian town of Slovyansk, is speaking out in protest.
“They asked us to leave; they said, ‘If you want to live, leave,’” Babai said last month in an interview with the Russian newspaper Komsomolskaya Pravda .
“Being there became very dangerous; they are driving out the Cossacks,” he said.
“We were forced to leave.”
Source: The New York Times