The UN Charter came into force on 26 June 1945 at the beginning of the Cold War. Due to the almost universal effect of the UN Charter and the comprehensive regulation of the legality of the use of force contained in it, every development of the ius ad bellum since then has to be viewed in close connection with interpretation of the Charter. At the same time, the political role of the United States changed from that of an actor in a multipolar system to that of a decisive state (one of the two poles) in an international system now considered as bipolar.

The Truman Doctrine

Just as earlier doctrines, the doctrines of US security policy during the Cold War period are first of all a reaction to a situation perceived by the US as a concrete threat. In the case of the Truman Doctrine it was the developments in the Greek Civil War in 1943–7 which led to the formulation of the doctrine.

Already in October 1943 at a time when large parts of Greece were still under German occupation, armed confrontations between the Peoples' Liberation Army (ELAS), belonging to the communist controlled National Front for Liberation (EAM), and the National Democratic Army (EDES) were taking place. The degree to which EAM/ELAS received support from the Soviet Union and Yugoslavia is still disputed to this day. A British intervention on the side of EDES in December 1944 solved this conflict temporarily.