The Common Security and defense Policy (CSDP) of European Union has formally established for more than 20 years but just achieved a little progress during these two decades. 21 years later after 1992, the stage of the European Union's role in the security and defense area is still stop in a primary level. It can be easily seen that the European Union lacking the ability to intervene international conflicts. Without a powerful military force, European Union just has a little voice in security issues. For example the EU's weak response to the Yugoslav war and the failure of CSDP in Iraq War. Why such a phenomena are constantly presented? Why an effective common defense system was hardly to build in such a highly integrated area? And why the CSDP developed a lot in late 1980s but bog down after the end of Cold War? Personally, these questions are interesting and worth studying, so this thesis will be centered on them. Below I will list some hypothesizes to explain the reasons and try to prove those hypothesizes with the case study.