US-backed Kurds were told Euphrates was a 'natural boundary' that they should not cross
When the US Navy shot down a Syrian Su-22 fighter-bomber over Syria the Russian forces in Syria proclaimed they would begin to track US aircraft west of the Euphrates as targets.
We have now learned that the Russians have seen the Euphrates as a "natural boundary" between Russian-backed and US-backed forces in Syria for a while now. A senior YPG commander told al-Monitor that Russians were upset with US-backed Kurdish forces when they crossed over to the western side of the Euphrates:
The YPG commander, responding to claims that the United States will deploy to Tabqa, also mentioned the Kurds' meeting June 17 with the Russians at Syria’s Khmeimim air base, saying, “The United States above all coordinates with Russia. They bargained with Russians, not with the Kurds, about Tabqa. Two forces drive politics in Syria: the United States and Russia. We are working for the integrity and democratization of Syria. That is why we are ready to fight at any corner of Syria.
Russians are criticizing Kurds not because of Raqqa, but Tabqa. Two days ago, we sat down with them at Khmeimim to ease their concerns. They don’t mind [us coming] down to the Euphrates but reject our crossing to the south of the river. We came down 25 kilometers; they told us the Euphrates is a natural boundary we shouldn't cross.
But this is a boundary between the United States and Russia. I said to them, ‘Don’t draw a boundary. We also want to reach Damascus. To draw a boundary is a scenario to divide the land. You are drawing such a boundary.’ The Russians were surprised by our reaction. They have to understand that our problem is not only autonomy for the Kurds.”
The SDF crossed the Euphrates in March in a (rather impressive) US-orchestrated operation near the Syrian town of Tabqa (exactly where the Syrian Su-22 would later be shot down). The first to cross Lake Assad which forms behind the Tabqa dam was a group of US Special Forces, after which Marine Corps helicopters transferred hundreds more SDF fighters. After a bridgehead was thus established many more SDF fighters and equipment were ferried across the next day on barges and rafts.
After crossing the Kurdish fighters eventually took the nearby Tabqa dam and city. Initially it seemed Tabqa was important to SDF as a springboard from which to isolate Raqqa city (the target of their Raqqa offensive) from the south, however the SDF was then extremely slow in actually moving eastwards from Tabqa to cut off Raqqa from its hinterland across the Euphrates.
The SDF is expected to complete the encirclement of Raqqa from the south any day now—however, the battle inside the city has been already underway for three weeks now.
On the other hand by controlling Tabqa the SDF controls the nationally-significant Tabqa dam and power plant (actually the SDF now controls all of Syria's three dams on the Euphrates — Tishrin, Tabqa and Baath).
It also put the SDF in control of a stretch of the main highway along the Euphrates which would have been extremely useful for the Syrian army in its march in the densely-populated river valley.
Whether by design or not, the US-directed operation to ferry the SDF across the Euphrates has had the effect of limiting the benefit of SDF's Raqqa operation for the Syrian army, and to frustrate its plans of continuing to march straight along the river.
The Syrian Su-22 which was downed June 18th by a US F/A-18 was attacked near Tabqa. US claimed the Syrian jet dropped bombs "near" the SDF, while Syria maintained the plane had been on a mission against ISIS. A scenario which would be consistent with both of the accounts is one where the Su-22 flew against ISIS but became lost. The US immediately downed the plane without engaging the deconfliction line with the Russians.
Tabqa is an ethnic Arab town, the SDF is a mainly Kurdish force.