This paper analyzes critical energy infrastructure in Southeastern Europe through the theoretical framework of neoclassical realism in international relations. The new international reality is defined by great power politics and geopolitical rivalries that shape energy flows and patterns of dependency. In this context, energy transition in the region derives from strategic calculations aimed at reducing vulnerability, creating alternative corridors and strengthening alliances in line with the particular interests. The central hypothesis posits that international pressures from great powers determine the framework for the development and protection of critical energy infrastructure, while outcomes depend on domestic intervening factors such as institutional capacities and internal political-economic conditions. Neoclassical realism provides the tools to explain why countries exposed to similar external constraints adopt divergent strategies of energy security. The methodological approach combines analysis of Southeastern Europe with a comparative examination of state responses to systemic pressures. Particular focus is placed on the influence of the United States and Russia in shaping regional dynamics. The argument is twofold. It first shows how great power politics accelerate the transformation of energy systems in Southeastern Europe, and then explains how the interaction of external and domestic factors ultimately shapes the character of critical energy infrastructure.