One of the Americans that went to fight for ISIS was killed by the FSA while he fought for ISIS. http://www.nytimes.com/2014/08/27/world/middleeast/american-fighting-for-isis-is-killed-in-syria.html?_r=0
Furthermore, plenty of articles show Assad would put the squeeze on the FSA bombing them and not ISIS.
There are about 5 different sides fighting over there, maybe quite a few more.
“ISIS and the New ‘Army of Conquest’ in Syria Are Headed for a Showdown”
We don’t need to agree with Pat Buchanan, some here might take issues with what he has written but he just wrote:
“While few would mourn the passing of the Assad dynasty, there is a problem: If Assad falls, a slaughter of Christians will follow and the battle for control of Damascus will be between the Syrian branch of al-Qaida, the Nusra Front, and the crazed terrorists of the Islamic State.”
http://buchanan.org/blog/our-next-mideast-war-syria-15984
:Reports coming out of eastern Syria Monday revealed that several factions within the Syrian opposition force known as the Free Syrian Army (FSA) have pledged services to the Islamic State, the group formerly known as the Islamic State of Iraq and al-Sham (ISIS). Sources and eyewitnesses said that the FSA has handed over its weapons to the Islamic State in large numbers.”