So, at one time the Moderates in Syria had a clear opportunity for victory. A US policy decision was on the table to arm, train, and support the Moderates to contribute to the goal of toppling Assad. Someone talked the Administration into believing that was a bad idea, because we would just end up arming militants. Turkey asked to go in and handle things, and we stopped them. NATO/EU tried to come up with a intervention, and we stopped them.
(As a side note: Russia is buddies with Syria, so we were trying to play nice with Putin by being nice to Syria - fat lot of good that did.)
Some of the people fighting Assad got desperate for aid and turned to militants for help. A flow of arms, fighters, and leaders from the chaos the US helped create in Iraq gave the militants muscle. These militants got organized, became the IS, and were not bashful about attacking the Moderates as much as Assad. The Moderates were beaten back in a two front war. No one paid attention until the IS grabbed land in Iraq, because they astutely took advantage of an opportunity presented. In doing so they gained credibility, more arms, and attracted more fighters to their cause.
The Administration finally started to notice that they may have made a mistake, and that sometimes doing nothing is worse than the pain of doing something.
I have seen several reports that conclude that the Moderates have lost, with few meaningful enclaves remaining. In a recent major battle/last stand against Assad, IS attacked their flank and rear and the Moderate opposition was largely shattered.
It is now we are going to arm and train the Moderates to win in Syria and face off with the IS... Moderates that have already lost the war they wanted to fight, and we are going to ask them to fight a war they probably don't want to fight. This situation is going to take a lot more will and investment from a lot more sources to clean up.
In my opinion March of 2003 was the worst mistake in US foreign policy history. And we have been in a disastrous tail spin ever since with mistake on top of mistake, and it is all coming home to roost.
