The United States does not want to get dragged into another intractable Middle Eastern conflict. Americans are tired of war. My colleagues Michael Gordon and Mark Landler have revealed how Obama blocked an attempt last summer by Hillary Clinton to train and supply weapons to selected Syrian rebel groups.

Nor does Obama want to find himself in the business of helping Islamist extremists inherit a Syrian vacuum. The opposition coalition is divided and lacks credibility. But the net result of these concerns cannot be feckless drift as Syria burns. Senator John McCain was right to say here that, “We should be ashamed of our collective failure to come to the aid of the Syrian people” and to answer a question about how to break the impasse with two words: “American leadership.”

An inflection point has been reached. Inaction spurs the progressive radicalization of Syria, the further disintegration of the state, the intensification of Assad’s mass killings, and the chances of the conflict spilling out of Syria in sectarian mayhem. It squanders an opportunity to weaken Iran. This is not in the West’s interest. The agreement that Assad has to go is broad; a tacit understanding that it is inevitable exists in Moscow. The Turkish foreign minister, Ahmet Davutoglu, spluttered in justified incredulity at the notion the opposition would sit down with a regime that has slaughtered its own.

It is time to alter the Syrian balance of power enough to give political compromise a chance and Assad no option but departure.