I know this is gonna sound cliche but... yeah, this is the US's fault.
See, there are three different religious... groups, shall we say, in Iraq (the names of which escape me at the moment), on of which is a majority, with the other two being large minorities. When the US instituted a 'democracy' of course the first thing that they did was to run elections, of which this majority won. This president, which is a part of the majority, of course, made sure that all the officials that he put in place were of the same majority, while influencing the police to investigate and imprison the politicians of the two minorities. Slowly, this meant that the other two minorities had little say in how they were governed. They took this as far as they could, and then decided that enough was enough, so they began taking their land back by force.
One of the minorities moved through lands occupied by their people like a knife through butter, with the Iraqi army running from them, because, well they didn't really care about this land. But as soon as the rebels reached lands held by the people that supported the government, they stopped. Why, because with no support, as well as the army now fighting tooth and toenail to keep these lands, they wouldn't get very far. The second minority suddenly decided that they wanted a piece of the action too, and started taking their own lands back as well.