Chairman of the Joint Chiefs of Staff General Martin Dempsey, the highest-ranking officer in the U.S. military, sent a letter to Congress laying out all the options for U.S. intervention in Syria: train, advise, and assist the opposition; conduct limited "stand-off" strikes; establish a no-fly zone; establish buffer zones; and control chemical weapons. (Details about each can be found at the link.)
Then he explained why intervening in Syria is a terrible idea (emphasis added):
All of these options would likely further the narrow military objective of helping the opposition and placing more pressure on the regime. We have learned from the past 10 years, however, that it is not enough to simply alter the balance of military power without careful consideration of what is necessary in order to preserve a functioning state. We must anticipate and be prepared for the unintended consequences of our action. Should the regime's institutions collapse in the absence of a viable opposition, we could inadvertently empower extremists or unleash the very chemical weapons we seek to control.
I know that the decision to use force is not one that any of us takes lightly. It is no less than an act of war. As we weigh our options, we should be able to conclude with some confidence that the use of force will move us toward the intended outcome. We must also understand risk -- not just to our forces, but to our other global responsibilities. This is especially critical as we lose readiness due to budget cuts and fiscal uncertainty. Some options may not be feasible in time or cost without compromising our security elsewhere. Once we take action, we should be prepared for what comes next. Deeper involvement is hard to avoid. We should also act in accordance with the law, and to the extent possible, in concert with our allies and partners to share the burden and solidify the outcome.
Elsewhere in the letter, he explains that for a no-fly zone "Estimated costs are $500 million initially, averaging as much as a billion dollars per month over the course of a year. Impacts would likely include the near total elimination of the regime's ability to bomb opposition strongholds and sustain its forces by air. Risks include the loss of U.S. aircraft, which would require us to insert personnel recovery forces. It may also fail to reduce the violence or shift the momentum because the regime relies overwhelmingly on surface fires--mortars, artillery, and missiles."
Next time you hear an American politician calling for a no-fly zone in Syria, remember, that person is suggesting that intervening there is a prudent place to spend up to $12 billion.