EU’s Common security as the principle and policy started in the 1950s with European Defence Community (EDC) but it never came into existence. Since then, the development of the EU had focused largely upon economic aspects, while the security aspects were sidelined. Until the 1980s, the term security and defence were not in the treaties until 1992. Both Western European Union (WEU) and Common Foreign and Security Policy (CFSP) were immediately subjected to the test during the Yugoslav War of Dissolution. Unfortunately, the war exposed the lack of solidarity within the EU. Throughout the war, the EU and its members were not able to put an end to ongoing atrocity when the assistance from the US was not available. The war provided the impetus for the EU to enhance its own ability in the realm of security. In 2008, Kosovo, a former Yugoslav territory, decided to unilaterally declare its independence. This action immediately caused the split within the EU on the issue of recognition due to the lack of legitimate UN resolution. Yet, the EU’s members, including countries that disagreed with Kosovo’s unilateral independence, showed its solidarity by adopting EULEX Kosovo, a civilian mission under the pretext of ESDP. Thus, as shown in this study, the solidarity can be best understood through the lens of Rational Choice Institutionalism.