“If you look at the heart of the Middle East, where the U.S. once was, we are now gone—and in our place, we have Iran, Iran’s Shiite proxies, Islamic State and the Russians,” added Mr. Crocker, now dean of the Bush School of Government and Public Service at Texas A&M University. “What had been a time and place of U.S. ascendancy we have ceded to our adversaries.”…

The Syrian deployment, in particular, has given Mr. Putin the kind of Middle Eastern power projection that, in some ways, exceeds the influence that the Soviet Union enjoyed in the 1970s and 1980s. Already, he has rendered all but impossible plans to create no-fly zones or safe areas outside the writ of the Assad regime—and has moved to position Russia as a viable military alternative that can check U.S. might in the region.

“What Putin wants is to establish a sort of co-dominion with the U.S. to oversee the Middle East—and, so far, he has almost succeeded,” said Camille Grand, director of the Fondation pour la recherché stratégique, a French think tank.

Russia’s entry has been welcomed by many in the region—particularly in Iraq, a mostly Shiite country where the U.S. has invested so much blood and treasure—because of mounting frustration with the U.S. failure to roll back Islamic State.