Inspired by the 2011 coalition action in Libya, Shahryar Minhas and I recently developed a new agent-based model of third-party intervention in civil wars. If you are attending the ISSS/ISAC conference in Chapel Hill, you will get a chance to hear about the project this afternoon. If not, our slides are here.
We model civil war as a gambler's ruin problem. Rebels start out with a fixed proportion of territory, always less than or equal to fifty percent. Depending on their relative strength, the long run expectation is that they will lose, just like you will lose to the "house" if you go for broke at a Vegas casino. Intervention can change the game, though: it would be like winning on black and half of the red spaces on a roulette wheel. Our model is a bit more detailed than that (particularly with how we model states' decisions to intervene) but that's the gist.
Here are the long-run probabilities of beating the government, given a certain percentage of territory occupied and a certain strength ratio, based on the gambler's ruin:
And after 1,000,000 simulations, here is how intervention affects conflict duration:
So the takeaway is that we predict intervention to increase conflict duration; but if the outcome is more desirable for the intervener, it may be worth it.