Modern human conflicts, such as those ongoing in Iraq, Afghanistan and Colombia, typically involve a large conventional force (e.g., a state army) fighting a relatively small insurgency having a loose internal structure. In this chapter, we adopt this qualitative picture in order to study the dynamics – and in particular the duration – of modern wars involving a loose insurgent force. We generalize a coalescence-fragmentation model from the statistical physics community in order to describe the insurgent population, and find that the resulting behavior is qualitatively different from conventional mass-action approaches. One of our main results is a counterintuitive relationship between an insurgent war’s duration and the asymmetry between the two opposing forces, a prediction which is borne out by empirical observation.