All these terror attacks Paris, Nice, Tunisia, Minnesota and now New York are being caused by IS, Daesh, NOT by Assad but by IS!
The only effective ground forces in Syria to fight this evil organisation, are the regime forces under Assad. The west had the Kurds but they withdrew their support for Syrian Kurds and ruined that special relationship to keep Turkey happy. The rebels can't be trusted so that only leaves the regime forces.
That means the west needs to cooperate with Russia and Iran to retake Syria from the growing IS. The west needs to ignore advice from Turkey.
A deal could be made that the west will support and arm Assad so long as he allows humanitarian relief into Allepo and takes the fight to IS.
These terror attacks are becoming a frequent problem and the only way to stop them is to support Assad.
A huge anti IS force will be needed including Iranian troops and Russian troops and NATO troops would be even better!
Regime forces can take Raqqa from the south with extra regime forces transported to Iraq to move in from the East and Airbourne regime forces parachuted into Northern Allepo province and Affrin to move in on Raqqa from the west liberating Al Bab and Manbij.
Once Raqqa has fallen the regime will control a line stretching from the Iraqi border to Affrin. They can then move North to liberate Kobane and Haskah.
Forces then move south liberating Deir Ezzor and areas around Palmyra. This would be like D day.