The risk of going beyond lethal aid to establishing a no-fly zone to keep Mr. Assad’s planes grounded or safe zones to protect refugees — options under consideration in Washington — is that we would find it hard to pull back if our side began losing. Given the rebels’ major recent setbacks, can we rule out using air power or sending in ground troops?

Yet the sum total of risks — higher oil prices, a widening war — also provide Syria (and its patrons, Iran and Russia) a motive to negotiate. If Mr. Obama can convince Iran that he is serious, and is ready to back up his new promise of aid with additional forces, Iran and Russia will know the risks: Mr. Assad could lose his regime, and most likely his life. Higher oil prices would cost China, which has blocked anti-Syrian initiatives at the United Nations, dearly…

The formula for diplomacy is clear: a cease-fire agreement; a United Nations presence; departure of foreign fighters; disarmament of Syrian fighters; international supervision of Syria’s military; a peaceful exit for Mr. Assad, his family and key supporters; a transitional government; and plans for a new Syria.