It was not wholly unreasonable for the U.S. to think Russia could be a partner in the war against ISIS. Yet there was always a fundamental disagreement at the root of that idea. The Obama administration insists Mr. Assad must step down to allow a negotiated end to the country's four-year civil war. Russia, by contrast, says that only Mr. Assad can bring stability to the country and that supporting his government is a prerequisite for peace. Moreover, Mr. Putin has refused to distinguish between various the armed factions fighting the Assad government, calling them all terrorists — including the so-called moderate opposition groups funded by the U.S.