Among the first clues that Russia was mobilizing for a military offensive in Syria were requests Moscow began making in mid-August for permission to cross other countries’ territory with more and larger aircraft.

“We were getting the word the Russians were asking for inordinate overflights,” a senior Obama administration official said, referring to reports from U.S. allies receiving the requests. Russia was seeking clearance for not only cargo planes but also “fighter aircraft and bombers” that Syrian pilots had never been trained to fly, the official said. “It was clear that something pretty big was up.”

But despite that early suspicion — which only intensified as Russia then deployed fighter jets and teams of military advisers — the United States seemed to be caught flat-footed by the barrage of airstrikes that Moscow launched last week.