Assad had no problem in supporting “Sunni” jihadists when they killed US troops in Iraq.
Of course, this raises the question of whether the left hand of our foreign policy knows what the right hand is doing. When Saddam was overthrown, the Shia majority took over in Iraq. This gave Iran far more influence in Iraq than it ever had. If having Iran-allied people in power is so terrible, then why were you and others so enthusiastic about putting them into a position of power in Iraq? Alternatively, if you can deal with an Iran-allied government in Iraq as preferable to Sunni militias, why is an Iran-allied government in Syria so much worse?
I don't have much of a dog in the Shia vs. Sunni fight, except to repeat the fact that the worst acts of terror committed against Americans and Europeans have been by Sunni, not Shia, fundamentalists.