In a letter to Senate Armed Services Committee Chairman Sen. Carl Levin (D – MI), Joint Chiefs of Staff Chairman Gen. Martin Dempsey warned that establishing a no-fly zone would be an “act of war” and would cost an average of $1 billion a month to maintain.

And that’s just the low-end figure. Dempsey warned US planes could be shot down by Syria’s anti-aircraft systems and that would require US deployments into Syria to try to recover the pilots.

Even then, a no-fly zone might not have any serious impact on the civil war at any rate, since very little of the fighting actually involves aircraft, and the Syrian military relies overwhelmingly on ground forces to fight the rebels.

Dempsey’s letter was in response to the requests by Levin and Sen. John McCain (R – AZ) for details about possible military “options.” McCain was holding up Dempsey’s nomination for a second term as joint chiefs chairman pending a response.