This article is, unfortunately, a bit behind the times. (Links to relevant articles are highlighted in blue, below.)
The U.S. is now having second thoughts about an air war over Syria, ever since Russia moved in their S-300 Air Defense Systems, the first time these systems have been deployed outside of Russia. In addition, the Russian missile corvette, Mirazh, left Sevastopol on Thursday to join Russian warships in the Mediterranean. Two sister ships, carrying Kalibr long-range cruise missiles, were expected to arrive in the Mediterranean this past Wednesday. Following these moves, Russia warned the U.S. that, if the U.S. were to attack Syria, Russia would shoot down its aircraft; and, further warned that Russia is was fully aware of the location of all opposition "operation rooms" and associated personnel within Syria.
One reason the U.S. appears to be backing off is that, although the U.S. is superior in terms of number of aircraft, cruise missiles, etc., this equipment is in many ways no longer a match for Russian air defense systems. An excellent analysis of why this is so is provided in the following article:
Why American Military Doctrine Is Doomed for Fail - Oct. 2, 2016 - Federico Pieraccini - Strategic Culture
There is also now some thought as to whether, once Russia has all its air defenses systems in place, Syria will decide to place its own no-fly zone over Syria; and, ban all U.S. and U.S. allied aircraft from entering Syrian air space.