This article seeks to answer this question by raising the question of why the European Union, despite its success in economic integration, has not been very successful in political-security integration and achieving a single voice in the international arena; by dividing the challenges facing the European Union's common foreign and security policy into three categories: structural, internal, and external challenges. In a short answer, it can be said that an examination of the existing challenges shows that the most important obstacle to the full realization of the European Union's common foreign and security policy is primarily due to the insistence and emphasis of member states on preserving national government structures such as national sovereignty and national interests. The result of the emphasis on preserving national state structures in the supranational arena is a unique situation that leaves the European Union suspended between a confederal and a federal structure. This fluid structure has been fully manifested in the process of decision-making in various policy areas. Despite the influence of other factors mentioned in the article, the conflict between national (national sovereignty and national interests) and supranational (limitation of national sovereignty and adjustment of national interests to common interests) approaches at the European Union level is considered the most important obstacle to the full realization of the European Union's common foreign and security policy.