Officially, Turkey considers itself at war with the Kurdistan Workers Party (PKK), a war which has lasted decades. Increasingly, however, PKK is being very vaguely defined, and includes Kurdish groups in several countries which Turkey argues are all basically PKK.
Doing this has allowed Turkey to go after limited PKK forces who have crossed the border into Iraq or Syria, but as lines blur, it’s tough to argue that Turkey isn’t just at war with Kurds in general at this point, particularly in Syria, where the PKK is virtually non-existent, and Turkey is after the YPG claiming they are the same group.
It is this interest in warring with the YPG, and with Kurds as such, tha5t will be a big focus of Turkey’s President Recep Tayyip Erdogan on his upcoming visit to the US, where he will express outrage at US military support provided to the YPG to fight against ISIS, arguing that amounts to backing a terrorist group.
The US has mostly tried to keep Turkey placated with assurances that the YPG won’t use US arms against them, but at this point the US has to have troops along the border just to keep Turkey from attacking YPG territory, and it’s hard to magine that, when Turkey finally does come over that border after the Kurds, they’re going to consider US arms off limits.