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For two years, the rebels in Al Qusayr held out against the forces of Syrian President Bashar al-Assad. Then in April the regime, supported by fresh fighters from the Lebanese militant group Hezbollah, renewed its attack on the mountain town overlooking the Lebanon border.

But it wasn’t Hezbollah that made the difference. It was the relentless bombardment by Assad’s air force that shattered the rebel defenses — killing 80 opposition fighters and sending the survivors into retreat. Assad’s jets and helicopters, unleashed against rebels and civilians alike in mid-2012, have proved a decisive force in the now 30-month Syrian civil war.

Because of this aerial onslaught, the rebels have begged the United States and its allies in the North American Treaty Organization to enforce a no-fly zone over rebel-held areas concentrated in Syria’s mountainous north. Barring that, opposition leaders have an alternative proposal: Washington and its allies supply rebel fighters with the weapons they need to defend a no-fly zone on their own.

But Washington says these weapons pose a security risk. So it remains unlikely that the United States will offer them to Syrian fighters.

No-fly politics

No-fly zones, enforced by air-defense missiles and jet fighters, have developed into a favorite tool of international intervention. Twin U.S. no-fly zones in southern and northern Iraq protected ethnic and religious minorities from dictator Saddam Hussein in the 1990s and early 2000s. A NATO no-fly zone shielded civilians and rebels from Serbian forces in Kosovo in the late 1990s. In 2011, NATO flew top-cover for Libyan rebels.