For the first time in fifty years, U.S. and Russian military forces have engaged in direct combat. Soldiers from the two countries last clashed during the Vietnam War, when Soviet soldiers shot down U.S. warplanes with anti-aircraft weapons. Last week, on February 7th, the two powers met again, when U.S. drones, attack helicopters, and fighter planes struck a contingent of pro-regime fighters near Deir Ezzor, a Kurdish-held city in eastern Syria. As would emerge later, among those killed were up to a hundred Russian citizens who were fighting in Syria as private military contractors, a shadowy mercenary force whose presence in Syria is not officially recognized by the Kremlin.

Not long before the attack, the Russian contractors, fighting alongside members of pro-Assad tribal militias, carried out a strike against a compound held by Syrian Democratic Forces, a Kurdish-led army, backed and advised by U.S. troops. During the maneuver, the Russian contractors crossed the Euphrates River near the town of Khusham, breaching what Moscow and Washington have agreed is the dividing line separating their respective zones of authority. U.S. Special Forces embedded with the S.D.F. called in the ferocious response from the air.

According to investigations in the Russian press, as many as two thousand or three thousand Russian contractors are involved in military operations in Syria. Most of them are linked to a structure called Wagner, a company that has apparent ties to Yevgeny Prigozhin, a onetime St. Petersburg restaurateur who became close to Vladimir Putin in the early aughts. Prigozhin ended up with lucrative contracts to supply food to the Russian Army and has taken to overseeing the sorts of enterprises that the Kremlin finds useful but doesn’t want to manage itself. On Friday, the special counsel, Robert Mueller, indicted Prigozhin and twelve other Russian nationals for allegedly interfering in the 2016 U.S. Presidential election. He is widely linked to a so-called troll factory in St. Petersburg, where hundreds of people are paid to sit and create fake social-media accounts to foster discontent and confusion in political discussions online. Wagner was named by its commander, a former Russian special-forces officer named Dmitry Utkin, who is a fan of the German composer. In 2016, Utkin was photographed at a Kremlin ceremony in which Putin handed out awards for military valor.

Many of the Wagner mercenaries previously fought with pro-Russia rebel groups in eastern Ukraine; their motives run from financial necessity to a kind of ultra-patriotic enthusiasm. (According to the Conflict Intelligence Team, an online monitoring group, average monthly salaries at Wagner range from ninety thousand to two hundred and fifty thousand rubles, or about sixteen hundred to forty-four hundred dollars.) Wagner fighters tend to be drawn from a hodgepodge of experienced veterans, war adventurers, and committed nationalists. A field commander who fought in Ukraine and knew many of those who died in Syria told the Russian paper Moskovsky Komsomolets, “They believed they went to defend the Russian world on the edges of our sphere of influence. So write it like this: ‘They perished for the sake of their homeland, and for an idea.’ ”

It remains unclear why the Russian contractors attacked the compound in Syria held by the American-backed militia. An oil-processing plant in the area might have been the ultimate target—an objective with both mercantile and strategic appeal in the Syrian war. A report in Moskovsky Komsomolets went further, quoting a source in Syria who said “it was a purely commercial issue. It had nothing to do with war.” Another Russian paper, Kommersant, reported that the offensive operation had not been sanctioned by the country’s official military command in Syria and was viewed as a “dangerous self-initiative.” According to U.S. military officials, before launching the air strikes they used a Russian-U.S. hotline to ask if Russian forces were in the area, and were told that none had crossed the Euphrates—meaning that the Russian military command either was not aware of the movements of the private contractors or did not want to admit the presence of soldiers it does not openly acknowledge.

When I spoke with Noah Bonsey, a senior analyst at the International Crisis Group who covers Syria, he told me that the entire incident is “clouded in more ambiguity than usual, even by the standards of the Syrian war.” As he put it, “The key question is: Who ordered this foray across the Euphrates, and why? Was it a local decision? Did the Russian military command know about it?” He suggested that the oil facility made sense as a target because, in recent weeks, the Trump Administration has referred explicitly to control over the oil fields of eastern Syria as leverage in the ongoing conflict. This may have increased Russia’s interest in those same oil fields. At the same time, Kurdish forces are caught in a prolonged battle with the Turkish Army in Afrin, in northern Syria. The S.D.F. has moved some of its troops to aid in the fight, repositioning them out of Deir Ezzor. “Someone may have viewed that as an opportunity,” Bonsey said.

Russia’s official involvement in the war in Syria began with an air campaign that Putin launched in September, 2015. Since then, reports have circulated in the Russian press of undeclared, private Russian mercenaries fighting on the ground. Russian contractors participated in both battles for Palmyra. The liberation of the city is perhaps Russia’s chief propaganda victory in the war. Shadowy, undeclared mercenary forces are attractive for several reasons: they allow the Kremlin to maintain the fiction that Russia is fighting from the air only, giving Putin the opportunity to declare an easy victory and avoid public discontent over the loss of human lives during a dragged-out conflict. Last December, during a surprise visit to a Russian airbase in Syria, Putin said that the “task of combating the armed groups here in Syria” had been “largely resolved,” and made a show of ordering many Russian units to leave the country.

Over the last two and a half years, Russia’s intervention in Syria has served several of the country’s interests, as Putin sees them. It saved the Assad regime from defeat, holding off the spectre of a regime change, and secured Russia an undeniable and influential role in the geopolitics of the Middle East. Considering Russia’s isolation in the aftermath of the conflict in Ukraine, Putin must consider it a fortuitous turn of events. It has also allowed Russia’s military officials and war planners to test a new generation of Russian weapons and military strategy. In Syria, Russia deployed drones widely, and launched air strikes with weapons guided by the country’s GLONASS navigation system. “The use of private military companies is part of this process of trial and error,” Ruslan Pukhov, a leading defense analyst and the director of the Center for Analysis of Strategies and Technologies, said. The appearance of groups like Wagner on the battlefield in Syria represents a “complete innovation” in Russian military practice, he told me.

As with much in the defense sphere, Russian officials are convinced they are playing catchup with the West, particularly with the United States. The Kremlin began to consider the notion of private military companies in the years after the U.S. war in Iraq, where companies such as Blackwater and Triple Canopy took on outsized roles. But whereas in U.S. operations private military contractors tend to perform support and logistics functions—escorting supply convoys, guarding bases and top officials—the opposite seems true of Wagner forces in Syria. Many Wagner units carry out dangerous assault missions, and while official Russian special forces work alone, or in concert with Russian air power, Wagner mercenaries are given the task of embedding and coördinating with Syrian militia groups.