While there may be evidence of chemical weapons use by Syria, and while this may constitute grounds for US action against Syria, these are questions which the US Congress is constitutionally entitled to decide.
The American Constitution assigns the power to determine questions of war and peace to the elected bodies of Congress, and not to the President alone. While in emergency circumstances the President's unilateral actions can be forgiven, no case has been made that the use of chemical weapons by Syria cannot await a Congressional vote on a resolution authorizing force.
To allow Congress a vote on foreign policy is not a sign of weakness, but rather of confidence in our democratic system. Setting a precedent of unilateral executive action should be frightening to this Administration.