What is occurring in the region is a clash within a civilization; to enable IS to portray it as a conflict between civilizations – and itself as the true defender of Islam – would be a grave strategic mistake.
Moderate Syrian opposition forces and local Kurds could be part of such a multinational Sunni force, but they are not in a position to substitute for it. If such an expeditionary force cannot be formed, air attacks can be stepped up, thereby at least slowing IS and buying time to develop alternative strategies. Under such a scenario, IS would remain less a problem to be solved and more a situation to be managed.
Diplomacy cannot play a large role at this point. No solution can be imposed, given disagreements among the outside countries with a stake in Syria and the strength of both IS and the Syrian government. What diplomacy may be able to do is reduce, if not end, the fighting between the Syrian government and its own people, as the UN is attempting to do in Aleppo.
The biggest danger in 2015 may well be a widening of the regional crisis to Saudi Arabia and Jordan. Intelligence and military support for both countries will be essential, as will enhanced efforts to help Jordan shoulder its massive refugee burden. In this time of unprecedented turmoil in the Middle East, one of the region's basic rules still applies: No matter how bad the situation, it can always become worse.
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Richard N. Haass, President of the Council on Foreign Relations, previously served as Director of Policy Planning for the US State Department (2001-2003), and was President George W. Bush's special envoy to Northern Ireland and Coordinator for the Future of Afghanistan. His most recent book is “Foreign Policy Begins at Home: The Case for Putting America's House in Order.” https://www.project-syndicate.org/commentary/isis-sunni-crisis-management-by-richard-n–haass-2015-02