In just over a week, Syrian rebel forces seized much of Syria’s northwest. First, the rebels captured Syria’s largest city, Aleppo, then Hama and Homs. On Sunday, they entered Syria’s capital, Damascus, taking the city without a fight as government forces fled.

The United States sees the fall of Bashar al-Assad as an extraordinary chance to rid Syria "once and for all" of chemical weapons that killed or injured thousands of people in its civil war, a senior U.