Thoughts on Intervening in Civil Wars
It occurred to me that one might carry the argument a step further. If you are going to invade a country, the fact that it is engaged in a civil war seems like a point in your favor. The two sides are spending resources fighting each other that might otherwise be used to fight you; with luck, you might even succeed in allying with one side against the other. That, after all, is how the U.S. managed its quick and easy initial victory in Afghanistan. The Northern Alliance supplied the boots on the ground, the U.S. provided them with unlimited air power, and that was enough to reverse the outcome of the civil war and drive the Taliban from power.
What are the arguments on the other side? Why might the existence of a civil war make conquest harder rather than easier?
One possible answer is that it makes it harder if the invader is unwilling to pick a side. It is then in the difficult position of trying to keep members of each side from killing members of the other while getting along with both. Since the invader's troops probably don't speak the language or know much about the society they are largely dependent on local allies, none of whom can be trusted not to take the opportunity to use their position to support their side of the ongoing civil war.
In the case of Iraq, the U.S. allied with the Kurds--gave them enough of what they wanted so that they were willing, on the whole, to refrain from shooting at us and support us against other people who were doing so. But the U.S. was not willing to support either the Shia or the Sunni in their desire to rule the country to the exclusion of the other.
A second answer to the question of why a civil war might make conquest harder rather than easier is that it raises the stakes. No reasonable person thinks that the U.S. wants to drive the Iraqis out of the country and replace them with Americans. It's unlikely that the U.S. wants to rule Iraq for any extended period of time. But a reasonable Sunni might well worry about ethnic cleansing by Shia, or a Shia about ethnic cleansing by Sunni. And each group has good reason to worry that the other would like to establish itself permanently in power, reducing its rivals to at best second class citizens.
The higher the stakes, the more desperately people are willing to fight. One of the problems with an interventionist foreign policy that it gets the country following it into conflicts where the other parties have much more at stake, and so are willing to put up with much larger costs.
Consider the Vietnam War. People in the U.S. complained that the South Vietnamese were not sufficiently willing to fight. But in fact the Vietnamese, North, South, and Viet Cong, kept fighting after suffering casualty rates, relative to their population, enormously higher than the rates that drove the U.S. out of the war. The reason is obvious: What happened to Vietnam mattered a lot more to the Vietnamese than to us.