Mark Katz has an interesting post on his blog, which is repurposed from his 1982 book about Soviet military thought, but applies to Syria today. The operative part is here:
In wars between the people and a regime of extreme reaction, both radicals and moderates unite to fight the dictatorship, with each group hoping later to establish its preferred form of government (radical Islamist rule or some form of democracy). The radicals in such a civil war may well initially be a relatively small and weak group compared to the moderates fighting the dictatorship. However, the radicals stand a good chance of eventually coming to power despite their initially weak position. For while some external forces are supporting the dictatorship, others will support the radicals, making them stronger compared to the moderate opposition. When the dictatorship eventually falls, the radicals are often in a position to take power since they have received outside support from their allies while the moderates have received nothing. Either of these could come to power, and so external support of the radicals increases their chances of actually doing so. The Americans, of course, also have the opportunity of supporting the moderate opposition, but because of the rigidity of American thinking, the U.S. does not do this. This is an error that the radicals can take advantage of.
He adds several caveats, but he introduces the idea that this is not a two-player game but rather should be thought of as a three-player one. There are similar dynamics between Brezhnev-era communist struggles in the Third World and Islamist-tinged insurgencies in today’s Middle East, in that radicals were often the most organized and thus most poised to successfully overthrow a dictatorship because they enjoyed more outside support, while moderates tended to get sidelined by their external sugar-daddies (especially after the dictator was ousted). Should Assad fall tomorrow, one can imagine that Islamists would enjoy greater means to organize voters and rally Syrians around their cause than the moderate elements within the opposition, which are divided and underfunded.