"A no-fly zone is not a monolithic thing," Joseph Holliday, a fellow at the Institute for the Study of War who focuses on the Syria crisis, said at a United States Institute for Peace event last month.
Holliday said that there is a "spectrum" of military options within a no-fly zone, from a full-scale air campaign to finding other creative ways to restrict the Syrian air force's capability. Still, Holliday, who tracks Syria's military capability, warned that the Assad regime seems to be keeping some military fighter jets in reserve specifically to deal with any possible foreign threat.
The Syrian Air Force is "not on its way out," said Holliday, who added that Syria retains "one of the densest air-defense systems in the world."
Administration officials have also questioned whether a no-fly zone would be an effective way to help the opposition defeat Assad and stop the slaughter of civilians.
"In Syria, when you have a situation where regime forces are intermingled with opposition forces and they're fighting, in some instances, block-by-block in cities, that's not a problem you can solve from the air," said Rhodes.
Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey told reporters in April that he doubted a no-fly zone could be effective in limiting civilian casualties
"About 10 percent of the casualties that are being imposed on the Syrian opposition are occurring through the use of air power," said Dempsey, who noted that the remaining 90 percent are by direct fire or by artillery. "So, the question then becomes: If you eliminate one capability of a potential adversary, will you be inclined to find yourself in a position to be asked to do more against the rest?"
Dempsey said the United States needs to be careful not to get drawn deeper into the conflict without having clear military objectives. If a no-fly zone was ordered, Dempsey said, the Pentagon would have to factor in the need to knock out Syrian air defenses and develop a search-and-rescue plan for any U.S. fighter pilots that could be downed. Military planners also would have to consider the prospect that Syria might launch retaliatory attacks both within Syria and beyond.
"Now, none of these reasons are reasons not to take action," Dempsey said. "But they all should be considered before we take that first step."