François Heisbourg is a special adviser at the Foundation for Strategic Research, a Paris-based think tank.

Successful military interventions are sufficiently rare as to induce utmost caution when contemplating the use of force in Syria, a country as populous as Iraq or Afghanistan and no less divided along religious and ethnic lines. Yet the legal, political, strategic and military conditions for an international operation are being fulfilled, which is in turn creating an opportunity to bring down Bashar al-Assad’s bloody dictatorship and its regionally destabilizing repression.

In legal terms, Syria’s border violations have created a new situation, one in which the Russian and Chinese vetoes at the U.N. Security Council would no longer prevent the legitimate use of force by Turkey. These Syrian violations have been denounced in strong terms by NATO and an unusually unanimous Security Council; yet Syrian attacks continue, leading to daily exchanges of artillery fire.

Turkey’s security is further imperiled by an unabated flow of refugees prompted by the Assad regime’s mindless repression. The regime has simultaneously created the conditions for a sharp rise of cross-border attacks by Kurdish activists against Turkish army and police personnel. Turkey could credibly invoke Article 51 of the U.N. Charter, entitling it to exercise its right of self-defense. This would allow Turkey to request military assistance from its NATO partners. The burden of proof would then be on Russia and China, if those countries sought to override Turkey’s Article 51 claim in the Security Council.

Politically speaking, the United States is likely to recover room to maneuver after Nov. 6, whether President Obama is reelected or a president-elect Mitt Romney is working in tandem with the incumbent. Turkish Foreign Minister Ahmet Davutoglu has been clamoring for active NATO support, reminding the alliance of the high human cost of its slowness to intervene in the Balkans 20 years ago — a disturbing but compelling parallel. As with last year’s intervention in Libya, many NATO members would be neither willing nor able to support forceful action, but the ones that count the most, France and Britain, would probably join. The French president has been broadly supportive of Turkey’s approach to the Syrian crisis. Several Arab states (including Saudi Arabia and Qatar) may also participate.

From a strategic standpoint, the civil war in Syria is in a stalemate, with Assad’s forces unable to crush the rebellion and the insurgency militarily incapable of overthrowing the regime. A realistic objective of intervention would be to tilt the balance in favor of the rebel forces, to help expedite Assad’s fall. As in Libya, and unlike in Iraq, intervention would enable the rebellion, not be a substitute for it.

In military terms, this would be achieved by establishing a 50-mile no-fly zone along the Turkish-Syrian border. No allied aircraft would need to fly in Syrian airspace, as air-to-air and surface-to-air missiles fired from Turkish airspace and territory would have the necessary range to shoot down Syrian bombers or helicopters in the exclusion zone. Allied AWACS radar aircraft, operating well out of range of Syria’s extensive air defenses, would provide full real-time information on any regime aircraft approaching the no-fly zone.

The zone would include Aleppo, which means the regime’s bombardment of Syria’s largest city would cease. Its fall, along with unimpeded access to logistical support from Turkey, would give the insurgency the upper hand.

And with no boots on the ground, this intervention would not require an exit strategy.

Such an intervention is becoming desirable as well as feasible, for want of a better option. Letting the civil war fester will lead to further destabilization in Syria and the wider region — some of which is already visible in Lebanon and Jordan — and a radicalization of the conflict. Providing the rebels with weaponry, notably anti-aircraft missiles, raises the dread prospect of blowback, given our inability to control the ultimate destination of such transfers.

Of course, even an intervention of the kind suggested here won’t guarantee a positive and stable outcome in Syria, any more than the overthrow of Moammar Gaddafi meant that milk and honey were to flow in Libya. But the alternatives are worse. We also know that a heavy “boots on the ground” approach — as in Iraq or Afghanistan — is to be strenuously avoided.