The first realistic, non-farcical option is the one that the president seemed to choose initially, when he launched limited airstrikes to rescue the embattled Kurds last month. This would basically be a strategy of containment and attrition, oriented around the current lines of battle in Iraq, in which we see if the Kurds and those Iraqi Army units that didn’t collapse can push the front westward, see if a post-Maliki government can woo local Sunni leaders, and use our air power to degrade the caliphate’s fighting capacity while letting its internal weaknesses degrade it from within.
The trouble with containment is that it would leave the Islamic State in control of a great deal of territory (with more beheading videos, no doubt) for months and years to come. Hence the administration’s pivot to Syria; hence the strategic dream palace that is our arm-the-rebels strategy.
The cold reality, though, is that defeating ISIS outright in Syria will take something more substantial than dropping a few bombs in support of a few U.S.-trained moderates. Either the American military will have to intervene in force (including with substantial ground troops) or we’ll have to ally, in a very un-American display of machtpolitik, with Bashar al-Assad. Both options may have supporters within the Republican Party. Many hawks seem ready to send in ground forces, and John McCain has explicitly argued that we should be willing to go to war with both Assad and the Islamists at once. From Rand Paul, meanwhile, you hear what sounds like a version of the ally-with-Assad approach, albeit couched in somewhat ambiguous terms.