The essence of the diplomats’ case, made in an internal memo, is that no peace deal is possible if the Assad regime is not confronted with the threat of military force. They were careful to advocate only the use of weapons like cruise missiles that would keep Americans out of the range of Syrian retaliation. They also rejected the idea of a large-scale American invasion.

But what if the “limited” airstrikes did not work? And however calibrated the operation, would it not inevitably draw America into another Middle East morass and, quite possibly, a military confrontation with Russia? Moscow is playing a double game in Syria by giving lip service to diplomatic efforts while conducting airstrikes that have allowed Mr. Assad to regain the upper hand on the battlefield.

A no-fly zone that could offer a safe haven for civilians from Syrian and Russian air power could also be problematical. Research by Micah Zenko of the Council on Foreign Relations shows that airstrikes are responsible for only a portion of the deaths; most are caused by shootings, mortar, artillery and rocket attacks. A truly protective no-fly zone would have to be quite large and extend to areas where there would be considerable risk of confrontation between American planes and Russian and Syrian planes.

There is also the matter of the legal basis for an American intervention. Mr. Obama has no United Nations Security Council resolution or authorization from Congress to justify military action against the Assad government. Some lawyers, like Harold Koh of Yale University, a former State Department legal counsel, suggest there is a case to be made for humanitarian intervention, but administration officials say they don’t see a basis for that in international law.

Russia remains of critical importance to any peace effort. Moscow’s support, as well as Iran’s, has allowed Mr. Assad to dig in his heels and resist compromise. Some administration officials still hope they can persuade President Vladimir Putin that he has much to lose by continuing to support Mr. Assad, not least by further alienating Sunni Muslim countries like Saudi Arabia and forcing Moscow to commit more troops and weapons into defending Mr. Assad. A complete collapse of the cease-fire, followed by increased support by the Saudis and others for their proxies on the ground, would risk even greater bloodshed.