This thesis will address the issue of legitimacy within international security, with a focus on the use of force by states. Using military force against other actors in the international system will initiate a debate on its perceived legitimacy by several different audiences. This investigation uses the Regional Security Complex Theory of Buzan and Wæver and the assumptions of Idealism to instigate the analytical framework on legitimacy. This thesis will analyse the role and importance of legitimacy and its potential influence it, as a concept, can have on the political agenda of states and organisations. It will also identify the multitude of factors which affect the perception of legitimacy. Moreover, it will analyse how multilateralism can address the different audiences demand for legitimacy as a doctrine in the modern world. The investigation will also show how the shortcomings of the international system limit the successful regulation of state actions in regards to the use of force.