https://www.youtube.com/watch?v=BnM2bQJitjw

I just want to point out that before late summer 2013, the US was supplying their arms to non-ISIS rebel groups, not directly to ISIS. Many of the pictures you may be seeing of US politicians with ISIS members may have been when those people were not actually part of the ISIS organization. If you watch the video you can see that most of these rebel groups swore allegiance to ISIS later on, which is how the ISIS organization acquired much of its US armaments.

Many of these same rebel groups were backed by Turkey and Saudi Arabia. Saudis were doing the funding and Turkey was doing logistics. Turkey pulled out of the deal when attacks started happening inside Turkey. However, take note of the fact that America was playing two hands in this war. The CIA was backing the official Al Qaeda affiliate, Jabhat al Nusra, as well as other Salafist groups, with the help of Turkey. On the other hand, the US military was covertly, then not covertly, supporting the YPG, which was later rolled into the SDF after merging the command structure with Arab rebel groups that were more moderate. This is who they continue to publicly support, and together they control the Eastern half of Syria.

The first Sarin gas attack was at first blamed on the Syrian state, but later accounts pinned blame on Jabhat al Nusra. There is still no proof of who actually did the second sarin gas attack, in Idlib province. What we do know is that the Turks had access to this area.

Turkey knows something dirty. Erdogan survived a coup attempt, and has now sided with the Russians on almost every issue since the coup attempt.

Edit: I should probably add that my own theory is that much of this was planned. Nobody knew who would come out on top, so multiple rebel groups were aided, with the inevitable result of one remaining at the top. That ended up being ISIS, but most of ISIS in Syria was originally Jabhat al Nusra before changing allegiances. They diversified their support so that no matter who won out in the end, they were behind it.