Social simulation models from computational social science are beginning to provide significant advances in terms of implementing more complex social, human, and natural dynamics that are characteristic of how countries operate in the real world. In particular, increasingly realistic agent-based models can improve capacity for early warning, understanding, and prediction. The MASON RebeLand model presents three innovations over earlier models: (i) an explicit polity model with politically complete structure and processes; (ii) social and natural model components within an integrated socio-natural system; and (iii) generative dynamics where insurgency and the state of the polity (stable, unstable, failing, failed, and recovering) occur as emergent phenomena under a range of social and environmental conditions. Three scenarios are demonstrated, showing stable, unstable, and failing polity conditions. The MASON computational system for agent-based and network modeling also permits additional experiments and extensions.