Since 2011, Syrian crisis has evolved into one of most complex regional and international conflicts, marked by the collapse of central authority of Assad regime and the emergence of non-state and quasi-state actors. In this context, foreign support particularly from regional powers such as Turkey and Israel has played a pivotal role in shaping battlefield dynamics and political developments. This paper focuses on the post-Assad period and the rise of a new successor state under leadership of Ahmad al-Shar’a (Joulani) in northwestern Syria, offering a legal analysis of the successor state’s responsibility for consequences of prior foreign support to non-state actors. From the perspective of international law, study examines the principles of continuity of state obligations and the responsibility of successor states regarding past commitments and their effects. The article also explores responsibility of supporting states through lenses of concepts such as effective control and international responsibility, analyzing the legal implications of their actions. It further assesses the challenges related to legitimacy, international recognition, and successor state’s engagement with non-state actors, as well as the opportunities for consolidating authority and assuming responsibility. Findings indicate that, despite existing constraints, successor state bears legal responsibility for the consequences of foreign-backed interventions, and that supporting states also hold direct accountability for actions of their proxy groups. This study adopts a descriptive-analytical method, drawing upon international legal sources, United Nations documents, and a case-based examination of post-Assad Syria, contributing meaningfully to the understanding of successor state responsibility and foreign support in modern armed conflicts.