"We don't restrict potential US action to a specific geographic space," the official added, say they would act against ISIS wherever they believe is "necessary to defend the United States."Though it was the fall of territory in Iraq that got the administration on the intervention bandwagon against ISIS, the group has at least as much territory in northern and eastern Syria, where it has been the most successful of Syria's rebels.

US involvement in Syria against ISIS would put it in a position of aiding the Assad government, despite months of throwing arms and money at various rebel factions. It would also add to anti-US sentiment among regional Sunnis, since it would amount to the US attacking the largest Sunni rebel faction fighting against two Shi'ite-ruled nations.