The other most notable examples of no-fly zones were those imposed by the United States and its allies over northern and southern Iraq following the 1991 Gulf War. Operations Northern Watch and Southern Watch (later, Southern Focus) aimed to prevent Saddam Hussein's air force from attacking Iraq's Kurdish and Shiite minorities. (Iraqi Mig and Mirage fighter planes were used in the 1988 gas attack on the Kurdish city of Halabja, which killed up to 5,000 people.)
Unlike Deny Flight, the Iraqi operations were never specifically authorized by the U.N. Security Council. The United States, Britain, and France claimed authority under Security Council Resolution 678, which stated that member states could use "all necessary means" to ensure that Iraq complied with its post-war disarmament obligations. However, many observers felt there was no basis in international law for the zones and the debate over their legality continues to this day.
The mechanism for enforcing a no-fly zone depends on the situation and the country doing the enforcement. In the case of Iraq, the zones were monitored by AWACS surveillance planes that would contact allied fighter jets, which flew regular missions, if a violation was detected. The operation was relatively effective -- very few violations of the zones were recorded between the end of the Gulf War and the U.S. invasion of Iraq in 2003 when it was finally lifted -- though human rights abuses against the southern Shiites by Iraqi ground forces continued. For those who argue that a no-fly zone would hasten Qaddafi's departure from power, it's also worth considering that Saddam Hussein ruled under one for over a decade. But the Iraqi experience also demonstrated the dangers of no-fly zones: In a 1994 incident, two U.S. Blackhawk helicopters were shot down by American F-15 fighters after being mistaken for Iraqi aircraft.
In the case of Libya, nearby Italy has suggested that it might allow its military bases to be used to stage the enforcement of a no-fly zone. The U.S. has its own airbase in the country as well. The United States is also positioning an aircraft carrier off the coast of Libya, which a Pentagon spokesman said will "provide flexibility" for future military options. The main obstacle to imposing a no-fly zone on Libya may be political, rather than military: U.N. diplomats say that 15-member Security Council is unlikely to agree to a zone unless there's a dramatic escalation of violence by the Libyan Air Force, and this time, it's unlikely the United States or its allies have the appetite to go it alone.
In any case, while a no-fly zone could presumably prevent Qaddafi's planes from firing on protesters or rebel forces, it would do nothing to stop his ground forces and mercenaries from continuing their assault. Given the limited utility then, the U.S. and its allies must now decide if all the trouble involved in setting up a zone -- including inevitable questions of legality -- are worth the risk.
Thanks to Michael N. Schmitt, professor of public international law at Durham University Law School and former legal advisor to Operation Northern Watch.