Syria's conflict is the most violent to emerge from last year's Arab Spring, taking thousands of lives over the past 18 months.

After dictatorships fell in Tunisia, Egypt, Libya and Yemen, Syrian President Bashar Assad's government resorted to a brutal crackdown and mass killings to stay in power.

The United States has called for him to step down and has imposed economic sanctions. There is disagreement over what other steps the U.S. should take.

Assad long has helped Iran aid Hamas and Hezbollah, destabilizing Lebanon while threatening Israel's security and U.S. interests in the Middle East.

But extremists among the Syrian opposition, Assad's weapons of mass destruction and worries about Israel's border security have policymakers wary about deeper U.S. involvement.