WASHINGTON — A top Syrian rebel commander says that U.S. missile strikes could change the balance of the civil war in Syria. But even the Syrian opposition is worried about the political vacuum that might follow U.S. intervention and the possible collapse of President Bashar al-Assad.

Gen. Ziad Fahd, the commander of the “southern front” for the Free Syrian Army, urged in a telephone interview Wednesday that the U.S. and its allies attack six air bases and three rocket-launching batteries around Damascus. He said that if the U.S. takes out these targets, his 30,000 troops in the Damascus area “can launch attacks on the rest” of the regime’s forces in the south.

The rebel commander said that following an attack, his moderate wing of the opposition, organized in five sectors of the Damascus area, “is ready to take over government ministry buildings to ensure security and stop looting.” He said that the al-Nusra Front and other extremist organizations that are strong in northern Syria are relatively weak in Damascus, an assessment shared by other analysts.

This endgame talk may be overly optimistic, but it shows that the Syrian war is approaching a potential turning point. If the U.S. strikes, the rebels will push to seize the advantage, and the fighting could enter a decisive new stage. The U.S. hopes that if the opposition forces gain ground after an attack, the regime and its Russian allies would be more ready to negotiate a cease-fire.

Given the risks of an expanding conflict, this weekend’s G-20 summit may provide a last opportunity for the U.S. and Russia to agree on the framework for the negotiations to end the war that were begun in Geneva last year. New statements of support for a diplomatic solution have come this week from President Obama, Secretary of State John Kerry and, perhaps most significantly, Russian President Vladimir Putin. But the momentum toward U.S. intervention appears to be increasing, as congressional support grows for a resolution authorizing a military strike.

The Syrian commander’s comments were reassuring in the sense that he has a military strategy for the next stage of the battle. But Fahd offered little detail on three key issues for any transition: protecting Syria’s Alawite community from revenge killing if the regime falls; working with reconcilable elements of the Syrian army to maintain order and prevent the kind of chaos that followed the fall of Saddam Hussein in Iraq and Moammar Gaddafi in Libya; and preventing the future use of chemical weapons by the Assad regime in a desperate last stand.

These issues are crucial, so they’re worth special attention. “I don’t anticipate revenge killing,” said Fahd, adding that his forces would “look to the courts” to prosecute any crimes by the regime. That’s not enough; the opposition needs a specific plan. On working with the army, Fahd said he has informants within the military but no plans for future liaison. That’s also not adequate. And finally, Fahd said the regime “perhaps” might use chemical weapons again, and for that reason the U.S. should take out the stockpiles and command-and-control for such weapons. This would probably require the use of ground troops, which Obama has ruled out.

These endgame issues are addressed in a new report to the State Department prepared this week by one of the Free Syrian Army’s key strategists. He outlines what he calls the “Damascus plan” for “handling the power vacuum in case of a sudden Assad collapse.” The plan includes steps for securing the chemical facilities; providing security inside the city; protecting Alawites from reprisals; and working with the Syrian army.

“The Syrian army is essential to have stability in the capital,” notes the opposition memorandum. That’s because many Syrians, even those who oppose the regime, fear a political vacuum. The plan urges the U.S. to encourage “defection of full units within the Assad army” that can be integrated with the opposition.

As the clock ticks toward a possible U.S. military strike, Putin and Obama have a last chance to agree on terms for bringing together the opposition and responsible elements of the Syrian regime (including the army). Otherwise, U.S. military action should follow, even with all its uncertainties and potential problems.

Assad’s reckless decision to use chemical weapons may yet turn the balance of this war against him — on a chaotic and unpredictable final battlefield, if it comes to that, or at the negotiating table.

David Ignatius is a Washington Post columnist. His email address is davidignatius@washpost.com.