So far, he is finding few international partners willing to engage in a conflict that has claimed more than 100,000 lives in the past 2½ years and dragged in terrorist groups on both sides of the battlefield.
Only France is firmly on board among the major military powers. Britain's Parliament rejected the use of force in a vote last week.
Russia's foreign minister, Sergey Lavrov, said Monday the information the U.S. showed Moscow to prove the Syrian regime was behind the chemical attack was "absolutely unconvincing."
With Navy ships on standby in the eastern Mediterranean ready to launch missiles, Congress on Sunday began a series of meetings that are expected to continue over the next several days in preparation for a vote once lawmakers return from summer break, which is scheduled to end Sept. 9.