This thesis examines how the European Commission’s discourse has evolved between 2019 and 2025 in response to geopolitical disruptions such as the COVID-19 pandemic and the Russian invasion of Ukraine. The study investigates how the Commission constructs legitimacy through the green transition for a security agenda themed around resilience, security, and strategic autonomy. Drawing on a governmentality framework and legitimacy theory, the thesis conducts a chronological comparative document analysis of the European Green Deal communication and the Strategic Foresight Reports published between 2020 and 2025. The findings show a clear discursive shift from sustainability as an environmental and social objective towards sustainability as a strategic tool for managing dependencies, geopolitical risks, and future uncertainties. This suggests that the green transition has become a form of governance in which sustainability is used to legitimize security-oriented and anticipatory policy interventions. The thesis contributes to debates on EU governance by showing how changing geopolitical conditions reshape both the meaning and political function of sustainability.