Well, if by "they", you mean Turkey, I'm not so sure I'd frame it that way. The Kurdistan issue has been around for a really long time. The basic problem is that with the Kurds, you have a cultural group who have been traditionally discriminated against, and want their own statehood. Unfortunately, that geographic region includes parts of modern-day Turkey, Iraq, Syria and Iran. Given that the aforementioned states have been pretty robust in prohibiting the creation of a Kurdistan, that stayed more or less stable for years. Then the first cracks opened up in 1991 when we kicked the Iraqis out of Kuwait. This created a humanitarian crisis in northern Iraq which the U.S. established a safe-zone to facilitate relief operations. Turkey had been entirely happy with Saddam keeping his Kurds in check, and now they were looking at a significant Kurdish population no longer under the direct control of a nation-state intent upon keeping them suppressed. Moreover, it was the U.S. that was the driving force behind all of it, and as a NATO ally they (Turkey) were in a bind. Eventually this led to an autonomous Kurdish rule in northern Iraq, which made Turkey nervous and also started stirring the pot for Turkish Kurds to press for their own autonomy -- probably eventually leading to their holy grail of full independence at some point. Then Syria fell apart, the U.S. started arming Kurdish militias to do most of the fighting against I.S., the Turkish military's (mostly secular) top brass got executed following a failed coup, and finally Donald Trump deciding that standing by allies is for suckers -- and we're where we are now.