The “defeat effect” is a socio-political phenomenon where an external attack or perceived threat triggers a “rally-around-the-flag” effect, increasing government support, followed by erosion due to military setbacks or perceived failures. In the spirit of cliophysics, which applies quantitative methods from physics to historical and social events, this study defines measurable features to characterize the defeat effect and predict government stability outcomes. Analogous to astronomical observations (e.g., stellar position and brightness), we quantify the phenomenon using nine metrics and two contextual attributes across 16 cases, including 15 democratic regimes (e.g., the USA, the UK, Israel, Ukraine, France, India, US-Vietnam War 1968, Israel-Second Lebanon War 2006, UK-Iraq War 2003, Weimar Germany) and 1 authoritarian case (Russia) obtained using AI agent Grok. A new case (Argentina-Falklands War 1982) tests the predictive model, demonstrating the cliophysical approach’s potential for forecasting political outcomes.