Obama's strategy of arming "moderate" Syrian rebels to defeat PISSI is fantasy, and even confirmed Syrian enemies of Assad say so.

Even a right-wing commentator acknowledges this.

So did Obama, not long ago. (Search the article for "fantasy".)

On the other hand, if the Turkish army is willing to fight PISSI in Syria, it could win. Since Turkey is mainly Sunni, Syrian Sunnis who fear Assad would massacre them for their sect could trust Turkey not to do that. (Turkey is no great defender of human rights, but it may be good enough for this.) This might enable the Syrian Sunnis to reject PISSI.

Other things could go wrong: the Turkish army might start fighting Kurds in Syria, for instance. But there is hope of avoiding that.

There are many problems with Obama's plan for dealing with PISSI.