In 1998 Sweden accepted the Treaty of Amsterdam which contained a development of the common foreign and security policy (CFSP) towards a European security and defense policy (ESDP). This thesis aims at studying the decision making process behind the Swedish foreign policy decision to further develop its security policy within the European Union. The purpose is to explain which factors and conditions that did affect and influence the Swedish foreign policy decision. The research applies foreign policy analysis, a theoretical perspective that analyzes both domestic and international conditions using different levels of analysis. The research methods being used has been qualitative as well as quantitative, complemented by interviews, since the research is a process tracing case study. The thesis conclusion is that the Swedish decision is explained by several factors. Conflicts in Yugoslavia resulted in a development of CFSP. Sweden considered that the Treaty of Amsterdam didn’t challenge the military policy of non-alignment and at the same time Sweden had the opportunity to influence the content of the treaty. It was also important that the attitude of the Swedish public opinion facilitated a Swedish participation in a European security and defense policy.