In 1945, the creation of the UN aims to prevent a new world war. But we are also witnessing a very strong period of tension between the United States and the USSR. The USSR and the United States were allies against Nazism but at the end of the Second World War, these two hyperpowers (they are called: the two blocks) will oppose each other without ever directly confronting each other because of the fear of nuclear. This period is called the Cold War, it will last from 1947 to 1991. Problem: How did the Cold War manifest itself? How does the opposition between the 2 blocks take place? What are the reasons for the end of the Cold War and their consequences. In February 1945 (when the war was not yet over) the YALTA conference took place. Roosevelt (United States) Churchill (United Kingdom) and Stalin (USSR) decide on a new organization of the world: occupation of German territory by the winning powers - denazification of Germany - organization of free elections in liberated countries - creation of ONU. In 1945 ; at the Potsdam conference the victors decide on the new borders of Europe. Gradually tensions arise between the winners. The Russians are accused of wanting to spread communism in the countries of Eastern Europe which they liberated from Nazism. For their part, the Russians find it difficult to accept the world domination of the United States (atomic bomb) From 1947, the 2 former allies are suspicious of each other: it is the end of the great alliance against Nazism and the beginning of the Cold War. The cold war will gradually spread between the 2 "BIG" => the world is divided into two parties.