In this dissertation, I investigate how periods of systemic transition shape the foreign policy behavior of middle powers. I am primarily interested in answering the following questions: why do middle powers become more assertive during international structural change, and what explains variation in the extent and direction of their assertiveness across regions? I conceptualize systemic transition as a distinct structural condition marked by the erosion of hierarchical order, growing uncertainty, and the strategic recalibration of hegemonic power. Unlike stable periods of polarity, systemic transitions are defined by fluidity and differentiation, offering both constraints and opportunities for state action. My central argument is that middle powers tend to assert greater autonomy when the dominant power, faced with a rising challenger, reallocates strategic resources and selectively withdraws from regions of lower strategic importance. This disengagement weakens hierarchical constraints and creates space for regional actors to pursue more assertive roles. However, the erosion of hegemonic control is not uniform across all regions. While the hegemon reinforces authority in core areas vital to its strategic rivalry, peripheral regions experience a decline in oversight, allowing middle powers located there to recalibrate local balances of power with fewer risks of punitive backlash. This spatial variation offers a more nuanced understanding of when and where assertiveness is viable for middle powers. To evaluate this argument, I employ a qualitative process tracing methodology. This approach enables detailed within-case analysis to identify the sequence of events and intervening variables that link systemic-level change to state-level behavior. The dissertation examines two case studies: Turkey and Iran. These cases are selected for their distinct regional positions and varying relationships with the dominant power, allowing for a comparative analysis of assertiveness under different conditions. This study advances our understanding of international order by conceptualizing systemic transition as a distinct structural condition that reshapes hierarchy and subsystemic dynamics, with significant implications for middle power behavior. Policy-wise, it offers a framework for anticipating where and when middle powers are likely to assert autonomy and how dominant powers may respond strategically.