ISIS is seeing some significant setbacks in Syria. Its de facto capital, the Syrian city of Raqqa, is under serious threat from Kurdish (YPG) troops. ISIS "is barely surviving in Syria," Yasir Abbas, an associate at the private research and consulting firm Caerus Associates, told me last week. "It is struggling to halt YPG advances and is out of low-hanging fruit [to seize]."
ISIS has recently lost some critical territory in northern Syria. To see how quickly that's happened, first look at this map of the battle-lines in Syria, from the always-excellent Institute for the Study of War, in late May. Pay attention to the purple (Kurdish) and grey (ISIS) blotches up north, especially near Tal Abyad (labeled as Tel Abyad on the map):
Now compare that to this second map, also from ISW, on June 19. Look how much territory Kurdish forces, in purple, have taken. Kurdish forces are also a lot closer to Raqqa (labeled ar-Raqqah), the ISIS capital:
By June 23, "ISIS lines of defense were pushed back to the gates of Raqqa," Rami Abdel Rahman, the director of the Syrian Observatory for Human Rights, told the Guardian.
Why did this happen? The immediate cause was, on June 15, Kurdish forces routing ISIS in the strategically important Syrian town of Tal Abyad — "a stunning defeat," as the Washington Post's Liz Sly called it. "Tal Abyad commands the major trade and smuggling routes on which the Islamic State has relied for its supplies from the outside world and, most significant, the flow of foreign fighters to Raqqa," she explained.
The Kurdish YPG represents a new kind of threat to ISIS in Syria. Well armed, experienced, and supported by US airstrikes, they were in a good position to push ISIS back. It helped that ISIS has also been fending off Iraqi government counteroffensives in western Iraq, stretching their forces thin.
ISIS's defeats near Raqqa are the result of "a combination of overstretch and increased YPG capacity due [to] the air support they are getting from the US," Abbas said.
Whether the YPG will press further towards Raqqa isn't clear: right now, it looks like they want non-Kurdish rebels to assist and perhaps take the lead in any assault on the city. If Raqqa is put under siege, it'd be a serious blow to ISIS: not a fatal one, but more evidence suggesting ISIS may have reached its military peak in Iraq in Syria — and would likely be headed toward slow, halting, but sure decline.