As outlined in his ISIS prime-time speech, President Obama now expects to arm friendly (to us) free-Syrian rebels so they can take the fight to ISIS. Thus negating any U.S. boots on the ground.
The problem with that thinking is that those rebels are already tied up in trying to wage war against President Bashar Assad's army. There efforts so far have been, at best, a stalemate. Now, they are expected to fight ISIS at the same time?
While arming the rebels might sound good and appear to be a workable strategy to the uninformed public, it just isn't a viable plan for destroying ISIS in Syria. Don't forget that, just a month ago, Obama belittled these rebels as being just a bunch of farmers, doctors, and pharmacists and not trained soldiers. Yet, now, he expects those same rebels to fight ISIS and Assad. But simply, despite maybe getting training and some weapons, these doctors/pharmacists/farmers, without any outside help on the ground, will be stretched too thin to fight two different battles; thus, weakening themselves on both fronts. As a result, Assad will be able to beat them back and ISIS will probably be able to finish them off completely. Nothing will be gained other than both Assad and, more importantly, ISIS will grow stronger.
This simple fact is why our U.S. military commanders like the chairman of the Joint Chiefs of Staff General Dempsey are saying we need "boots on the ground" to fight ISIS.
Arming Syrian rebels, key part of Obama's war strategy: http://www.foxnews.com/us/2014/09/15/arming-syrian-rebels-key-part-obama-war-strategy-gets-crucial-support-from/
Barack Obama rebukes (arming) Syrian (rebels as) ‘fantasy': http://www.politico.com/story/2014/08/barack-obama-rebukes-syrian-fantasy-109890.html
Dempsey raises possibility of involving U.S. combat troops in fight against Islamic State: http://www.washingtonpost.com/world/national-security/dempsey-raises-possibility-of-involving-us-combat-troops-in-fight-against-islamic-state/2014/09/16/8e13a742-3da1-11e4-b0ea-8141703bbf6f_story.html