Syria has become something like Spain in its 1936-39 civil war, i.e., a country that is not only carrying out a domestic struggle but has become a focal point for a much larger conflict. The USA should have learned from its experiences in Iraq, and later Libya, that fishing in troubled waters can bring with it very profound consequences. Try as they might, the pro-democracy movement, which was never under the leadership of any one organization, found it difficult to retain a commitment to non-violence. (Reports have circulated that both Assad and the anti-Assad forces have used chemical weapons.)
This was especially the case once a new factor entered the picture: Saudi Arabia and Qatar. As part of an effort by ultra-conservative Sunni theocracies to both crush a secular nationalist regime plus thwart the alliance between Syria and Iran, Saudi Arabia and Qatar began to supply militaristic elements within the anti-Assad coalition.
The other force to enter the picture against Assad have been the Sunni jihadists, some of which are associated with Al Qaeda.