If, in effect, Al Qaeda in Syria is moving from strength to strength, this is a profound challenge for American policy in Syria. Some observers fear that, unchecked, the rebranded group could use the Syrian conflict to construct the most powerful terrorist base since Al Qaeda lost its Afghan strongholds following the post-Sept. 11 downfall of the Taliban in Afghanistan. The conundrum for American policy makers is how to block the creeping ascendancy of Qaeda-linked rebels in the armed coalition fighting the dictatorship of President Bashar al-Assad.

The considerations behind American military action against Fatah al-Sham are complex and delicate. If the United States coordinates airstrikes with Russia, as Secretary of State John Kerry proposes, Charles Lister of the Middle East Institute warns that such action will “only serve to drive more young Syrians” into the arms of Fatah al-Sham and undermine moderate forces.

The key for the United States is to find a way to halt and ultimately reverse the influence of Al Qaeda, under whatever name, in the Syrian opposition. The first course of action should be to put the new group’s professed separation from Al Qaeda to the test.

The United States should lay out a series of benchmarks — building on the position shared by both Washington and the other Syrian rebel groups that Fatah al-Sham will be judged by deeds, not words — that would indicate a real break with Al Qaeda, rather than a rhetorical and tactical contrivance. These could include the renunciation of “takfiri” ideology (which brands other, non-jihadist Muslims as death-deserving apostates), the repudiation of Al Qaeda’s goals and methods, the abandonment of terrorism and a commitment to a nonsectarian future for Syria.

A precedent for this policy exists in the uneasy but apparently sustainable modus vivendi the United States has developed toward Hezbollah in Lebanon. The radical Shiite group is on the State Department-designated list of foreign terrorist groups, and it is illegal for Americans to provide it with any support. But the United States is not in an open conflict with Hezbollah, despite the group’s sending thousands of fighters to support the Assad government in Syria.

The benchmarks would operate in the full understanding that the former Nusra leaders are unlikely, and probably unable, to move toward such a moderate stance. The indications are that Mr. Jolani and his followers will remain committed followers of Al Qaeda and its broader agenda. But it would be essential to demonstrate their noncompliance to the other Syrian opposition groups in order to counter the extremists’ maneuver.

Al Qaeda in Syria emerged as a leading player in the Syrian opposition because it proved itself one of the strongest military forces in the resistance against the brutal offensives of the Assad government. But Nusra’s rise also owed something to the absence of effective international and American engagement with the moderate rebel groups. If the United States wants to ensure that terrorists are not the primary beneficiaries of Syria’s collapse, it should begin by calling their bluff and exposing them as unreconstructed fanatics.