Russians with Kurdish (YPG) fighters.
Last week Turkey pounded numerous Kurdish positions and groups across Syria and Iraq. Erdogan was basically testing the US to see how strong its commitment to its Kurdish allies is, and if possible to try and drive a wedge between them.
Americans did react telling Erdogan to lay off and rode around flying the US flag in Kurdish-held areas.
There was a catch however, Syria's Kurdish Afrin enclave was left as unprotected as ever.
Afrin area is in the north west where there is no ISIS presence. Also, while nominally in rebellion against the Syrian government, Afrin's biggest beef is with the Islamist rebellion against Damascus.
Being therefore useless against Assad and ISIS both, the US has ignored the Kurdish militias in Afrin, denying them even the lame material and political support that it bestows on the YPG-SDF militias in eastern Syria.
Rather than leave them completely at the mercy of Turkey, Russia has now stepped in. Russian troops have been sent north into Kurdish territory to fly the Russian flag.
This should help ward off further Turkish attacks which is good for the local YPG as well as for Syria's integrity.
Russia's willingness to react to calm things down is on the one hand laudable, but it also raises some questions. Are its soldiers in Kurdish-held Afrin observers, peacekeepers, or combatants? Or maybe human shields?
Are they there to fight alongside the Kurds to stop a further Turkish incursion into Syria, or just report on it if they observe one? There certainly aren't a whole lot of them. Barely enough to defend themselves, if that.
Afrin -- virtually surrounded by Turkey and rebels.
Comment: See also this South Front report on the deployment: