^{After the Second World War, the European countries, by reducing the differences, gradually took steps towards the goal of a common foreign and security policy, and in this direction, especially after the Cold War in line with a common security and defense policy. The decision-making place for the common security and defense policy is the European Council, consisting of the heads of member states who make unanimous decisions on civilian missions and military operations in order to promote global peace and security. The European Security and Defense Policy was recognized as the security and defense branch of the European Union during the EU summit in Helsinki in 1999, where its institutional framework was established. This path of evolution continued through subsequent summits until 2009, when the Lisbon Treaty renamed the "European Security and Defense Policy" to the "Common Security and Defense Policy." This policy aims to create operational capacity and capabilities by utilizing military and civilian assets and benefiting from the member states’ strengths. Despite the EU's achievements in developing and managing the Common Security and Defense Policy, it still faces challenges that hinder its optimal international performance. Among these challenges, we can mention the structure and decision-making procedure in the European Union, the Euro crisis, disagreements among member states, and nationalism. Especially since the member states in most cases act based on their national interests and do not consider the collective interests of the European Union. This research aims to examine how the decision-making process affects the international functioning of the Common Security and Defense Policy and the EU's role as a global actor. Based on this, this question has been raised: How does the decision-making process impact the international functioning of the EU’s Common Security and Defense Policy? The hypothesis is that decision-making based on unanimity leads to competition, complexity, slow progress, and, in some cases, delays, blockage, or vetoes of decisions. Using a descriptive-analytical method and the concept of EU actorness, this study first describes the structure of the Common Security and Defense Policy and its decision-making process and then explores and examines its challenges. Subsequently, by reviewing case studies of military operations and civilian missions led by the Common Security and Defense Policy, especially in the West Asia region, it addresses the main challenges and damages caused by the decision-making process that have affected the Common Security and Defense Policy and the EU's desired global role. Mentioning case examples such as Greece's blocking of EU joint decision-making in condemning China's human rights records and actions in June 2017, the veto by Cyprus against a joint decision to condemn human rights violations in Belarus in 2020, the lack of consensus among EU members to condemn the Trump administration's decision to recognize Jerusalem as the capital of Israel in 2017, Hungary's refusal to join other EU members in calling for a ceasefire between Israel and the Islamist group Hamas in May 2021 and the blocking of a joint decision on the U.S. invasion of Iraq in 2003 by some members indicated a tough test to build consensus and coherence in the EU's common security and defense policy, which ultimately failed. Similarly, when the EU intended to sanction Russia for its invasion of Ukraine, it encountered difficulties due to Hungary’s refusal to cooperate. In all these cases, the important issue is that a member, even a small country, is able to undermine the joint decision of the Union or to veto it altogether. This challenge is created due to the consensus-based system, which gives even small countries the ability to disrupt joint decisions and even veto these decisions. This poses a major challenge to the EU’s Common Security and Defense Policy, as a member’s veto shows that unanimity allows for external interference. Countries outside the EU that oppose a particular policy only need to influence one member state to achieve their goal. They must simply find the weakest link in the chain—and there are many weak links in this chain. Case studies reveal that most countries, when participating in missions and operations of the common defense and security policy of the European Union, prioritize their national interests over the EU’s collective interests. Thus, the institutional structure and decision-making process based on unanimity have, in practice, created divergence among the members. This issue has placed large and small countries on an equal footing, where smaller nations can easily come under the influence of external actors and challenge the EU’s cohesion. The cohesion of the common defense and security policy depends on how member states respond to international developments.  Experience has shown that if the responses are uncoordinated, the position of the European Union and its member states on the international stage will be weakened. All these factors have challenged the concept of EU actorness. In order to be able to compete with organizations such as NATO and the United Nations in the world, the European Union needs more cohesion and coordination among its members. Greater coordination and coherence will lead to greater credibility for the Union at the global level. But the voting system based on consensus has been able to damage the coordination and coherence of the European Union. Without consensus-based decision-making, external actors would not have been able to influence specific EU countries, and opposing countries would not have been able to pressure the EU by maneuvering their own interests. Without unanimity, internal disagreements would have decreased, and the EU could have made more coherent decisions and taken stronger positions in response to international developments. Therefore, the conclusion is that unanimity-based decision-making, by creating obstacles to the development of the EU's actorness, has in many cases prevented the fulfillment of the Common Security and Defense Policy's objectives. This issue has forced European countries to invest more at the national level, enter into international agreements and rely more on America and an organization like NATO. The set of these factors and approaches are not in line with the goals and plans designed for the European Union, and as a result, the European Union is placed in a process of further weakening and marginalization in important and security fields.}