The second major conflict in modern history began on September 1, 1939, when Nazi Germany invaded Poland, which would be conquered in a few days. The first phase of the war of conquest was marked by a succession of victories of the German troops and their allies (Italy and Japan, with Germany formed the Axis), who conquered all of Eastern Europe, occupied Belgium, the Netherlands, part of France and much of North Africa, dominated the central and eastern Mediterranean and launched an effective submarine warfare designed to isolate Europe from America, while Japan, which had invaded China years before, dominated the islands of the Pacific, vast regions of China, Burma, the Indonesian archipelago and even the Portuguese Timor and was approaching the British India.
The resistance to the expansionist policy of the Axis was first starred only by the United Kingdom, which with blood, sweat and tears (in the words of Winston Churchill) avoided being invaded, then the Soviet Union, which only managed to halt the German front at the gates of their cities (Leningrad, Moscow and Stalingrad) and in the Far East, China, which was in a long civil war but united against the nippon invader. Following the Japanese attack on Pearl Harbour, America abandoned its isolationist policy throughout the conflict and launched its military potential and its large industrial capacity.
After many months of German victories, the Allies, coordinating their efforts with the activities of the resistance organizations that were formed in the occupied territories began several counter-offensive attacks that put the Axis on the defensive and finally led to the defeat - the Soviet Union was able to break the siege of Leningrad and Moscow and counter-attacked from Stalingrad; American troops landed in North Africa and from there jumped to Europe (after Sicily and southern France and Italy); joint forces open a second front in Normandy (D-Day, June 6, 1944); the Americans invert the situation in the Pacific, freeing the Philippines and, jumping from island to island, dominating the waters and the skies and eliminate all the advantages of the Japanese forces. While these actions were happening, the resistance forces fixed the Axis troops at the rear of the front lines and diminished their mobility, destroying lines of communication and supply, with enormous human sacrifices (both combatants and killed hostages).
Despite their initial superiority that seemed insuperable, Germany was not able to withstand the impact of the counter-offensives, losing hundreds of thousands of men in decisive battles such as El-Alamein in North Africa or Stalingrad, USSR; simultaneously, the use of aviation in large-scale air strikes brough the destruction of both industry and roads and massive losses. The same happened to their Axis partners progressively weakened. In 1944, Germany surrendered, almost completely destroyed, socially disorganized, economically ruined, and its territory divided among the invaders; in August 1945, U.S. aircraft launched two atomic bombs on Japanese cities (Hiroshima and Nagasaki) causing tens of thousands deads and leaving the survivors with irreversible consequences causing great suffering, leaving stunned and horrified the world and hastening Japan's surrender, also exhausted. The arrival on the scene of this new weapon, whose true destructive potential before anyone accurately assessed, not only marks a profound change in the technological means of warfare, but also marks the beginning of the Cold War, since its launching was also interpreted as a sign of U.S. warning to its Soviet ally, out of the conflict with superpower status.