President Barack Obama wanted to approve military action against Syria to send a message to Syrian dictator Bashar al-Assad that the use of banned chemical warfare against defenseless civilians is unacceptable. Obama also wanted to prevent terrorists from potentially using these weapons to harm the United States.
After Bashar al-Assad was accused of using a deadly nerve gas on more than 1,400 citizens in Damascus, including a number of children, Obama declared that Assad's using or transporting such weapons within Syria's borders was "a red line." This line could not be crossed without American retaliation and intervention. There is an international law against the use of chemical warfare because these weapons are considered to be particularly heinous. Obama wanted al-Assad and others tempted to use chemical weapons to understand that the United States and the broader international community would not tolerate the mass execution of civilians in such an inhumane manner.
As president of one of the most powerful nations in the world, Obama desired to send an irrefutable signal to the United States' enemies that the nation does not make idle threats. In addition, Obama worried that some of Assad's chemical weapons might be procured by terrorists with motive to deploy them against the United States and its allies.