They paid a high price to find out.
As soon as the attack began American troops answered with artillery strikes and followed up with AC-130 gunships, fighter jets and Apache attack helicopters. The attackers had tanks, artillery, multiple-launch rockets and armored personnel carriers but no air cover. Their armor was destroyed in the first few minutes and they suffered grievous casualties. Some reports listed the number of dead Russian mercenaries at over 100 with another 200 wounded.
Intercepted audio transmissions (Transcripts here) reveal the extent of the devastation and carnage visited upon the attackers by the American forces who seem to have suffered no casualties themselves.
Although the Russian troops are said to have been independent contractors and not active military personnel this distinction is sometimes a fig leaf, as it has been in the Ukraine, that allows the Russian government to deny that it's their policy to engage in hostilities with non-belligerent forces. It also allows them to deny that their policies are getting Russian boys killed in acts of military aggression.
If Putin and the Russian government were behind the February assault it'd be most alarming that they'd be willing to use Russian troops to directly attack American forces. Why would they do this? Are they trying to precipitate a wider conflict with the U.S. in the Middle East? What would they have to gain? Yet it seems, if this WaPo report is correct, that somebody in the Kremlin gave the green light for some sort of undertaking in Syria:
A Russian oligarch believed to control the Russian mercenaries who attacked U.S. troops and their allies in Syria this month was in close touch with Kremlin and Syrian officials in the days and weeks before and after the assault, according to U.S. intelligence reports.Moreover, if this were a deliberate attack sanctioned by the Kremlin, having suffered a humiliating defeat will they be inclined to strike again at some point to try to vindicate Russian honor? It's hard to believe that the Russians would just accept this bloody nose without seeking some sort of revenge.
In intercepted communications in late January, the oligarch, Yevgeniy Prigozhin, told a senior Syrian official that he had “secured permission” from an unspecified Russian minister to move forward with a “fast and strong” initiative that would take place in early February.
On the other hand, there's some reason to doubt that the Russian leadership was directly responsible for this episode:
Not only is it difficult to see what the Russians would've hoped to gain by attacking a base where Americans could be expected to be engaged in combat, American officers were apparently in radio contact with their Russian counterparts before, during and after the battle. Perhaps the Russian military knew what was in the offing and was at pains to insure that the Americans didn't think that they were in any way responsible for the attack.
It may be that whoever gave the orders for the Russians to launch the raid may have been acting on his own and may have incurred the displeasure of the Russian authorities, but at this point that's just a guess.
As of the present neither the Russians nor the Americans have had much to say about the matter, which is, of course, a positive sign that may point to a desire on both sides to avoid escalation.
In any case, conflict between Russia and the U.S. is both highly unnecessary and highly undesirable. Hopefully, Mr. Putin sees it the same way.
UPDATE: As more is learned about what happened in this fight it may be that Russian casualties weren't nearly as high as originally reported. See here for details.