A mural in Belgrade, Serbia praises the Russian Wagner group and its mercenaries fighting in Ukraine, March 30.

Photo: Pierre Crom/Getty Images

Germany’s foreign-intelligence service recently intercepted secret messages confirming Russian mercenaries known as the Wagner Group played a leading role in the massacre in Bucha, Ukraine. For those who track the Wagner Group, this was expected. In recent years, it has become Vladimir Putin’s weapon of choice because it offers plausible deniability. Hiring mercenaries is a foolproof way to confound international laws prohibiting savagery in war. The Wagner Group has left a trail of atrocities everywhere it’s gone: Syria, Libya, the Central African Republic, Mali and now Ukraine. And other nations are following Moscow’s example.

It’s impossible to know for certain whether the Wagner Group’s brutality is the work of rogue warriors or Russian policy, but it is plausible it’s the latter. Coercion and terror are a time-tested imperial strategy for pacification, from the First Jewish-Roman War (66-73) to the Second Chechen War (1999-2000), during which Mr. Putin flattened Grozny. When the fighting was over, the city looked like Stalingrad after the Nazi’s five-month siege—post-apocalyptic, with only skeletal buildings left standing and scorched rubble burying unknown dead. Immoral but effective, Mr. Putin’s bombardment ended the war in Russia’s favor.

Ukraine should expect similar treatment. The mass graves in Bucha indicate how freely the Wagner Group can inflict terror. If Mr. Putin’s mercenaries do something worthy of disapproval, he can simply disavow them, as he did after the Wagner Group got shellacked by U.S. troops at the Battle of Kasham in 2018 in eastern Syria. Days after the battle, Russian Foreign Minister Sergei Lavrov dismissed it as “fake news.” Moscow’s protestations aside, American soldiers killed more Russians that night than any night during the Cold War, but the bloodshed didn’t escalate to World War III because both sides invoked plausible deniability. It’s unlikely they could have done so if Russian troops were involved. This ambiguity lets the Wagner Group act as an extension of Russia’s grim strategy and send a gruesome warning to states that might oppose Mr. Putin’s imperial ambitions without Moscow suffering the consequences.

Wagner mercenaries are generally proud of their work. I started speaking with them last year, when a member of the group first approached me because of my background as a former military contractor. This led to conversations with others. In general they remind me of other mercenaries I’ve interviewed; they do it for the money, adventure, profession of arms, or simply lack a life plan. What sets them apart from other mercenaries, aside from their superior lethality, is that many Wagner guys are also pro-Putin and support his vision of restoring Greater Russia. Most rank-and-file mercenaries don’t care about politics, but some Wagner contractors view their work as another way to serve the motherland.

For other Wagner mercenaries, the charm is wearing off. There seems to be a recognition among many that they are ultimately cannon fodder. They would rather chase lucrative contracts in the Middle East. Moscow prevents this in a typically Russian way. If Wagner personnel are caught talking to outsiders about their covert work, the Russian government can arrest them for being mercenaries, which is strictly banned under Russian law. The Kremlin hires them illegally, and then prosecutes them if they squeal. It’s a diabolical way of maintaining discipline.

International law can’t be easily used to bring the Wagner Group to heel. One would assume that the laws of armed conflict—binding treaties Russia has signed—would deal harshly with mercenaries, but the rules mostly ignore hired guns. One exception is Article 47 of the 1977 Geneva Protocols I, which defines and outlaws mercenaries but is almost unusable against the Wagner Group. The rule’s characterization of a mercenary is so restrictive yet imprecise that almost anyone can wiggle out of it. Wagner mercenaries fighting in Ukraine wouldn’t fit the definition because they are Russian and the protocols’ wording excludes anyone who is “a national of a Party to the conflict.” Moreover, the law stipulates that a mercenary is a nonstate combatant motivated primarily by the “desire for private gain,” which is difficult to prove in any circumstance. In 2005, the United Nations established a working group on the use of mercenaries, which has done nothing meaningful.

Even if a strong mercenary ban existed in international law, no one would be capable of enforcing it. World leaders wouldn’t authorize a foreign state to enter their countries and arrest people, and there’s no international consensus to empower a multilateral body like the U.N. to take up that role. Even if someone did show up to arrest Wagner Group members, there’s nothing stopping the mercenaries from simply opening fire on law enforcement. If they could be apprehended alive, the sort of international trials Wagner Group fighters would undergo are notoriously ineffectual and expensive. Having worked in war zones across Africa, I have never heard a society demand that hundreds of millions of dollars be spent on a Hague trial. After mercenaries devastate a community, people would generally rather take the money and rebuild their lives. And forget about sanctions. The Wagner Group and the oligarch the State Department identifies as its owner, Yevgeny Prigozhin, have been under U.S., U.K. and European Union sanctions for years. It hasn’t diminished their operations. (Mr. Prigozhin has denied he is linked to the Wagner Group.)

Mercenaries and atrocities have gone hand in hand throughout history. It’s one of their chief selling points. In 1209 Pope Innocent III hired a mercenary army for a crusade against the Cathars, a heretical sect in southern France, after the assassination of the papal legate Innocent had sent to counter their unorthodox beliefs. The papal forces crushed town after town until they came to the Cathars’s stronghold in Béziers. The mercenaries tore through the streets, killing Cathars and Catholics alike. Panic-stricken residents fled to the churches seeking sanctuary but received none. “Kill them all, God will know his own,” the replacement legate supposedly said. The quote may be apocryphal, but that’s what the mercenaries did.

The Wagner Group is part of a wider and worrying trend in international relations. The number of mercenary operations seems to be increasing and it’s because hired guns allow clients to wage war brutally with minimal political costs. Every time mercenaries get away with something—from assassinating the president of Haiti to springing billionaires from jail—it serves as an advertisement to future clients. Should this trend continue, we should expect more massacres and torture. The sort of violence perpetrated in Bucha may become a common facet of modern war.

Mr. McFate is a senior fellow at the Atlantic Council, a professor of International Security Studies at Georgetown, and author of “The New Rules of War: How America Can Win—Against Russia, China and Other Threats.”