If you've been listening just to the Kremlin and the Pentagon, you probably didn't know that Russia attacked American forces and their allies in Syria last week, suffering heavy casualties.

Yes, all sides admit that there was an incident at a U.S. base in Deir Ezzor. And that elements of the Syrian regime and Shiite militias participated in the assault. The Pentagon and Kremlin both acknowledge that Russian "mercenaries" participated, too. But the line for now is that those contractors had gone rogue, and Moscow didn't know anything about it.

When reporters asked U.S. Secretary of Defense James Mattis about the incident, he called the whole thing perplexing. "I have no idea why they would attack there, the forces were known to be there, obviously the Russians knew," he said. "We have always known that there are elements in this very complex battle space that the Russians did not have, I would call it, control of."

Now, it should be said that Mattis, a retired four-star Marine Corps general, is a very smart man. His perplexity in this case is probably what Plato called a "noble lie," a falsehood spoken by a leader to achieve a greater social good. If Mattis acknowledges the obvious -- that the Kremlin authorized a direct assault on a U.S.-sponsored base by non-uniformed personnel -- he risks an escalation spiral in Syria. Better to express bewilderment and give Russian President Vladimir Putin a chance to back down and deny culpability, which he ended up doing despite the heavy casualties suffered by his mercenaries.

But make no mistake: There is overwhelming evidence that those Russian contractors were working at the behest of the Kremlin. What's more, the Russians knew U.S. military personnel were in Deir Ezzor, which has been part of successive agreements to separate, or "deconflict," forces fighting in Syria.

Let's start with the fine reporting of my colleagues at Bloomberg News who discovered that the wounded mercenaries were flown out of Syria and treated at military hospitals in Moscow and St. Petersburg.

U.S. officials who monitor Syria tell me that there is no doubt that the Russian military knew all about the attack in Deir Ezzor. Evelyn Farkas, a former deputy assistant secretary of defense for Russia, Ukraine and Eurasia during the Obama administration, told me Thursday: "Any Russian mercenaries, whether they are in Ukraine or Syria, work for the Russian government."

This is not an accident, particularly for the contractor in question, Wagner. One of its leaders, Dmitry Utkin, is a former lieutenant colonel in Russia's military intelligence agency, the GRU. He and the firm have been closely tied to the oligarch Yevgeny Prigozhin, known as "Putin's chef" because he owns the Kremlin's food-service providers.

Contractors like Wagner are a key part of Russia's broader strategy of "hybrid warfare," a mix of kinetic and information aggression to advance Russian interests -- such as the deployment of fighters without uniforms that helped take Crimea from Ukraine in 2014.

"They help Russia obfuscate Russia's role in Syria," Matti Suomenaro, a researcher for the Institute for the Study of War told me this week. "In eastern Syria, the Russian Ministry of Defense can say, 'We don't know they were doing this.' But it's very likely this had some kind of direction from higher-ups in the Kremlin."

Finally, there is the strategic argument for why Russia would participate in the attack at Deir Ezzor. U.S. policy at the moment is a bit confusing. When Mattis and other U.S. officials publicly discuss the U.S. mission in Syria, they say only that it is to fight the Islamic State. So far, there is no official policy on whether the U.S. military's role includes countering Russian-Iranian efforts to help the Bashar al-Assad regime retake territory it lost in the civil war.

Add to this the mixed messages sent by the U.S. last month when it failed to stop Turkey from bombing the Kurdish-controlled city of Afrin. While the U.S. has attempted to end the Turkish assault through diplomacy, it has not offered to protect Kurdish fighters aligned with the People's Protection Group, or YPG, who remain in the city. The YPG are key U.S. partners in the campaign against the Islamic State. Indeed, Kurdish fighters stationed in Deir Ezzor in recent weeks have traveled to Afrin for the fighting, making the enclave a more attractive target for the Russo-Iranian alliance in Syria.

For a cynic like Putin, whose air force has bombed other enclaves of U.S.-supported rebels in Syria, the inability of the U.S. to stop a NATO ally, Turkey, from attacking another ally in Afrin is a sign of weakness. The assault last week on Deir Ezzor with mercenaries was a chance to again probe for a U.S. response.

The good news is that the U.S. response was swift and brutal. While there are no hard figures on casualties, some Russian press outlets reported that more than 200 Russian mercenaries were killed.

This brings us back to Mattis, and why he declined to directly blame Russia for the incident. "My guess is he said he was perplexed because he was sending a signal to the Russians: 'I am willing to give you a little time to cut this out, but don't do it again,'" Farkas told me. "And the Russians know they are playing with fire, if you look at how they are responding."

There is a downside, though, to this kind of noble lying. Considering that mercenaries like Wagner are a key part of Russia's broader strategy and tactics, it's also important for the U.S. to deny Moscow its plausible deniability. Russia needs to be told, going forward, that an attack by its mercenaries will be treated as an attack by its armed forces.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.