The rise of extremists had led to Russia’s intervention. Kerry said (at minute 26) that when Daesh, or ISIS, started to grow, the US watched and thought we could “manage” the ISIS situation, because it might push Assad to negotiate, but instead Putin came in.
Kerry says in the tape:
“The reason Russia came in is because ISIL was getting stronger, Daesh was threatening the possibility of going to Damascus and so forth. And that’s why Russia went in. Because they didn’t want a Daesh government and they supported Assad.
“And we know that this was growing. We were watching. We saw that Daesh was growing in strength, and we thought Assad was threatened. We thought, however, we could probably manage, that Assad would then negotiate. Instead of negotiating, he got Putin to support him.”