This study aims to analyze the mechanisms of social solidarity and national capacity-building processes in Iranian society during the 12-Day War with Israel (June 13-24, 2025). The theoretical framework integrates Durkheim's mechanical solidarity theory, Kriesi's political opportunity structure theory, and Lavergne-Saxby's social capacity-building approach. The research methodology is qualitative, based on documentary content analysis including examination of official statements, media reports, and social reactions. Findings indicate that external threats triggered rapid activation of collective consciousness and created cross-spectrum solidarity that united opposing political factions from Mehdi Karroubi to conservatives. The revival of Iranian national symbols such as the "Ey Iran" anthem and Iranian lion imagery alongside religious identity played a crucial role in strengthening national cohesion. Extensive public cooperation with security institutions and reporting of suspicious activities led to the identification and arrest of a significant number of infiltrators and the discovery of explosive materials, demonstrating the depth of social participation and sense of national responsibility. Additionally, resistance economy systems and essential goods distribution networks showed appropriate effectiveness, and societal resilience was revealed during communication restrictions following internet cuts. However, challenges such as human casualties, damage to nuclear facilities, weaknesses in early warning systems and passive defense, and generational differences in crisis perception were exposed. The research conclusion confirms that Iranian society possesses deep reserves of social solidarity that can be activated during crises, but institutionalizing these achievements requires intelligent management and long-term planning.