For Washington, the potential fallout of these scenarios is truly frightening. Chemical or biological weapons in the hands of Alawite vigilantes, Islamist terrorists or criminal gangs. Full-scale fighting along Syria’s borders. The release of pent-up ethnic and religious hatreds in Lebanon or Jordan. A renewal, after forty quiet years, of shooting between Syria and Israel. Military victory for what might eventually become the jihadist-dominated rebels leading to the establishment of Taliban-style rule in Damascus and the possible creation of a breakaway Alawite canton in the mountains of Latakia. Throw in weakness and division among western allies, a possible face-off with muscle-flexing Russia, and the wild card of how Iran may exploit the Syria crisis to press ahead with its own regional ambitions—and its nuclear program—and this is a witch’s brew for U.S. interests that would consume the energies of the president and could put any strategic pivot to Asia on hold for a decade.

Preventing these calamitous outcomes should be a high priority. But it is reasonable to ask whether prevention—in the form of outside intervention—will itself trigger some of these scenarios. Might it be better to let the current fighting take its course and not stir up the hornet’s nest even more?

The answer is no. Left to its own, the Syrian rebellion may eventually succeed in bringing down the Assad regime, but the key to preventing these negative outcomes is speeding up the pace of change. A slow, grinding conflict in which the regime continues its merciless but ultimately futile whack-a-mole strategy is the most likely backdrop for these nightmare scenarios. In contrast, swift and decisive action to hasten Assad’s departure is the best way to immunize against this set of terrifying outcomes. While Assad may unleash some of his fury in the face of assertive international action, chances are more likely that a clear display of resolve in support of the opposition is the key ingredient to fracturing his surprisingly resilient governing coalition and bringing the regime tumbling down.