Article in The Atlantic explains how the Saudis created ISIS and Qatar funded Jabhat al-Nusra with encouragement from the US while it supported the soon-to-be-doomed Free Syrian Army. The reporter here, Steve Clemons, is a totally reliable source.
But why oh why was the US so committed to destroying the Assad regime in Syria?
One word: Israel.
And it gets crazier still in less substantiated but still believable comments. Though it has predictably gotten out of hand, this was the US-Israel plan all along, to destabilize secular nationalist governments in the Arab peninsula with balkanized weak war lord states. As blogger Sarastro92 says in the comments to the article, this was part of Operation Clean Break to secure the realm for Israel by burning down the Arab neighborhood. Other comments go farther back, claiming Israel secretly promoted the Muslim Brotherhood to destabilize Arab states, as well as promoting Hamas to destabilize the PLO (which has been very well established). It was the Assad regime's dedication to destroying the Brotherhood (Israel's secret weapon) which made Israel so dedicated to destroying Assad. Further, Israel has long had secret cooperation with Saudi Arabia to foster radical Islam. From Israel's standpoint, this serves to discredit and weaken all its neighboring Arab states. Elsewhere I've heard that Israel wanted Iraq broken up into 3 states all along, and has long promoted a largely untrue narrative about Iraq's Maliki being a terrible sectarian.