First, Assad’s actions, however deplorable, are not a direct threat to U.S. national security. Many bad actors on the world stage have, tragically, oppressed and killed their citizens, even using chemical weapons to do so. Unilaterally avenging humanitarian disaster, however, is well outside the traditional scope of U.S. military action.
Second, just because Assad is a murderous thug does not mean that the rebels opposing him are necessarily better. As of May, seven of the nine major rebel groups appeared to have significant ties to Islamists, some of whom may have links to al-Qaeda and other terrorists. Their presence and power have only increased, according to media reports. We should never give weapons to people who hate us, and the United States should not support or arm al-Qaeda terrorists.
Third, the potential for escalation is immense. Syria is in the midst of a sectarian civil war, born of centuries-old animosities. We have no clear ally in this Sunni-Shiite conflict, and any “limited” and “proportional” strike could quickly get out of control, imperiling our allies and forcing us into the civil war.