TLDR Up Front: When ISIS first captured Palmyra it marked the high-water mark of its expansion in Syria. Now – significantly reduced in territory, ISIS has once again been able to gain ground in the south. Though it is nowhere near as powerful as it once was.
This all seems so familiar.
Full Context in the Back:
Two strategic elements this ISIS victory does highlight that might not be immediately apparent from the reporting.
A) The Syrian government *cannot* hold the country together with the ground forces it has. As it directs troops to retake Aleppo from the rebels (and this includes many reinforcements from Iran, Iraq and Lebanon militias and armies) it it loses the ability to counter another competitor in other areas and ISIS can take them. This is whack-a-mole-within-your-country. This is why ultimately any territorial division solution to the civil war, if there is one, will at best see a “Syria” that largely consists of the western third of the country along the coastal regions.
B) This is the first time I can think of where Russia made a concerted airstrike effort to impede ISIS from taking an objective. Reports of ~65 air strikes in a few days. Russian air power, such that Putin is willing to commit, is *insufficient* to overcome the strategic reality of the point above.