The Americans ecouraged and facilitated the "Arab Spring" which was a total flop. AS soon as you oust a dictator the fundamentalist jihadists take over and unending civil war complete with terrorist bombings galore ensues..
Assad created ISIS after the American invasion of Iraq by flooding arms to the Sunni insurgency in Iraq. He figured he could control it and that after it kicked out the Americans he could realize his dream is unifying Iraq and Syria with himself in charge. This was before the Syrian civil war.
Then the Syrian civil war happened, and his pet insurgency in Iraq merged with the Syrian opposition and with Al Qaeda.
Basically Assad lost control of his pet insurgency in Iraq and it turned around and bit him in the ass.