US officials are making clear that they intend to invade Raqqa really soon, with the ISIS capital scheduled to be attacks in the middle of the ongoing offensive against the city of Mosul. But while the Mosul invasion has a coalition of at least sort of friendly factions participating. No such coalition exists for Raqqa.
Indeed, the US has insisted that the Kurdish YPG will be the overwhelming majority of the force, because they’re the only force available in the area with enough fighters to realistically attack the city. Yet Turkey has been outright livid over this, and has repeatedly demanded that the Kurds not be allowed anywhere near Raqqa.
Turkish President Recep Tayyip Erdogan insists his country will be invading Raqqa, along with the YPG-held city of Manbij, very soon, and while they’ll doubtless also do some fighting against ISIS on the way, it’s pretty clear this is going to include some heavy fighting with the Kurds.
That’s likely to complicate, and potentially outright derail, the invasion of Raqqa, as the Kurdish YPG can’t fight ISIS and Turkey at the same time, and the US seems wholly incapable of keeping Turkey from launching attacks on the Kurds in the middle of anti-ISIS operations.