This chapter provides an introduction to the analytical and historical aspects of the concept of collective security in international law. Taking the examples of Italy’s invasion of Ethiopia in 1935 during the League of Nations and the complaint brought by Hyderabad against India at the very inception of the United Nations in 1948, the chapter traces the complex dialectics of normativity and concreteness in debates concerning collective security. Mirroring the normative and institutional dilemmas underlying the two cases of Ethiopia and Hyderabad, it is observed that the questions of “external threats” (the threat or use of force between States) and “internal threats” (what today is referred to as the “responsibility to protect”) were the two most controversial issues in the post-2004 UN reform process. The High-Level Panel’s recommendations should in this respect be seen as responses to a breakdown beginning in the early 1990s of the pragmatic compromise lying at the heart of the UN Charter. This can be seen in two directions: first, as a result of an instrumental autonomy argument which asserts that the sovereign right of self defence includes the right to carry out pre-emptive strikes; and second, as a result of an instrumental communitarian argument which views sovereignty as an anachronistic obstacle to humanitarian objectives and thus outweighed by the need to protect fundamental human rights. In order to counter these two trends, the non-instrumental or cultural aspects of international law are indispensible for any robust account of collective security in the relations between States.