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.”