Why insurgent organizations stay together over time and why they maintain the fight under stress are questions of major concern for our understanding of war duration, conduct, and outcome. Structural integrity is the property of an organization remaining as a single intact entity, while cohesion refers to the creation and maintenance of cooperative effort toward the attainment of the organization’s goals. Although closely related, the conflation of structural integrity and cohesion is problematic. The body of the paper is a structured comparative analysis of two cases, the Irish Republican Army and the Karen National Union. First, this analysis demonstrates that cohesion and structural integrity are discrete concepts that each helps us to understand part of why, and how, insurgent organizations keep up the fight. Thus, despite much recent emphasis on “spoilers” in the conflict literature, understanding why armed organizations stay together or fragment is not enough. We also need to understand what makes insurgents perform effectively in the organization’s interest. Second, it points to some limitations of existing theories of structural integrity and cohesion in insurgent organizations and allows the author to conclude by suggesting some fruitful avenues for future research on these dynamics.