Between 1939 and 1945, the Second World War saw the so-called Axis powers and the Allies oppose each other and, as had already happened to the belligerents of the First World War, fought over most of the planet. The conflict began on 1 September 1939 with the attack of Nazi Germany on Poland and ended, in the European theater, on 8 May 1945 with the German surrender and, in the Asian one, on the following 2 September with the surrender of the Japanese Empire after the atomic bombings of Hiroshima and Nagasaki.
It was the largest armed conflict in history and cost humanity six years of suffering, destruction and massacres, with a total estimate of deaths ranging between 55 and 60 million individuals. The civilian populations found themselves involved in the operations to an extent hitherto unknown, and were indeed the declared target of bombings, reprisals, persecutions, deportations and exterminations. In particular, the Third Reich carried out the Holocaust with engineering methods to annihilate, among others, populations of Jewish origin or ethnicity, also pursuing a policy of ethno-political reorganization of Central-Eastern Europe which envisaged the destruction or deportation of entire Slavic populations, of Roma peoples and of all those whom the Nazi regime deemed “undesirable” or enemies of the Aryan race.
At the end of the war, Europe, reduced to a pile of rubble, completed the process of involution that began with the First World War and definitively lost the world political-economic primacy, which was assumed largely by the United States of America. They were opposed by the Soviet Union, the other great superpower forged by the conflict, in a tense international geopolitical equilibrium which was later defined as the Cold War. The immense destruction of the war led to the birth of the United Nations Organization (UN), which took place at the end of the San Francisco Conference on June 26, 1945.