While the Pentagon is providing airstrikes in support of the Kurdish YPG virtually daily in its fights against ISIS, reports are that the Obama Administration is blocking other nations’ plans to provide heavy weapons to the group, in hopes of bolstering their fight.
The YPG is the dominant Kurdish fighting forces in Syria, and controls broad swathes of territory along the Turkish border on both sides of ISIS territory, including the cities of Hasakeh and Kobani. The group has been fighting ISIS heavily on both fronts.
This has led many in the US-led alliance to see the YPG is their most reliable allies inside Syria, and the US itself seems eager to back them with airstrikes at every opportunity. Directly arming them, however, is apparently a step too far, and the reason is likely Turkey.
Turkey considers the YPG a terrorist organization, and has openly threatened to invade Syria to stem their advances along their border. They have chastised the US for aiding the YPG to the extent that they already have, and the administration is likely trying to placate them by limiting the aid to airstrikes.
Weapons would be a much different matter, of course, as Turkey sees themselves as the inevitable next target of the YPG if they manage to defeat ISIS, and those weapons would be used against Turkey, a NATO member.
Oddly, the US hasn’t extended the same ban to Iraqi Kurdistan, even though they hold similar secessionist ambitions, and Iraq has similarly objected to the provision of arms to the Peshmerga. Despite this, US allies have regularly provided arms to them, and the current US military spending bill being debated would have the US do the same.