Weakening the Syrian government helps ISIS. There is no going around that
A report from Jane's security experts estimate that, from April 2016 through March 2017, 43 percent of ISIS fighting in Syria was against the Syrian Arab Army.
40 percent of their efforts went to fighting rival Sunni militants, particularly those who had signed up for the Turkish-led Euphrates Shield incursion into northern Syria.
The remaining 17 percent of their fighting was against the US-backed, Kurdish-dominated SDF coalition.
To be honest we are somewhat surprised by these numbers. If anything we were under the impression fighting between ISIS and the SDF was at least as bitter and intense as between ISIS and the Euphrates Shield.
Nonetheless, the London-based Jane's is a widely recognized authority on all things military, and as westerners they have little incentive to overstate the severity of fighting between ISIS and Syrian government forces.
“It is an inconvenient reality that any US action taken to weaken the Syrian government will inadvertently benefit the Islamic State and other jihadist groups,” said Columb Strack, senior Middle East analyst at IHS Markit [Owners of Jane's IHS].
“The Syrian government is essentially the anvil to the US-led Coalition’s hammer. While US-backed forces surround Raqqa, the Islamic State is engaged in intense fighting with the Syrian government around Palmyra and in other parts of Homs and Deir al-Zour provinces.”
Any further reduction in the capability of Syria’s already overstretched forces would reduce their ability to prevent the Islamic State from pushing out of the desert into the more heavily populated western Syria, threatening cities like Homs and Damascus, the analysis said.