We don't actually know that. From what I've seen, the Syrian government faction was maybe a third of the fighting force, but they had air power and the other factions were never able to unite for various reasons. The arms from the U.S. didn't make that much difference, and the war dragged on because the government didn't have a good way to end it without losing alot of soldiers in the cities (and massacring even more civilians). That left siege warfare and massive bombing as their only options, and both of those are slow and ineffective.