Defeating the Islamic State will be the easy part. The hard part will be securing the peace, making sure that the forces converging on Deir Ezzor don't start fighting among themselves. The stakes for Deir Ezzor could not be higher. The Iranians want an overland route to the Mediterranean. The Kurds want a buffer between Assad's forces and their territory further to the north. In some ways, the situation is like the end of World War II, when Soviet and American forces converged on Berlin.