In leaked audio between secretary of state John Kerry and Syrian opposition reps, Kerry says the US allowed ISIS to grow as leverage against Assad. Quite an admission. At the 26:30 mark:

“The reason Russia came in is because they didn’t want a daesh government and they supported Assad. And we know that this was wrong. We were watching. We saw that Daesh was growing in strength and we thought Assad, we thought however that Assad might then negotiate. Instead he got Putin to support him.”