I doubt that the Russians nor the Americans would risk a war over their proxy armies. But Russia has drawn their red line of a anything that flies "target zone" to the west of the Euphrates river and smack down the middle of Raqqa -- not very smart.
The Russians just de facto ceded a part of Syria away from Assad to an occupying army and local rebels -- that is really not smart.
Foreign armies fighting in Syria was always a bad idea. Should have dug a backfire trench and let the fire burn out to conclusion.
