The Second World War was a military conflict that took place from 1939 till 1945 and that involved almost all of the European countries, Japan, the United States, China and the European colonies in North Africa, Asia and Oceania.
The causes of this war were: the nationalist discontent, of Germany because of the lost of important territories due to the Treaty of Versailles and of Italy because it did not received the countries that it deserved ; the economic protectionism that was adopted in order to reduce imports and that caused the increment of tensions between countries; the expansionist policies that  were adopted by Germany, Italy and Japan to expand their territories ;and the appeasement policies adopted by Great Britain to maintain peace and tolerate those expansions.
This war was between two opposing sides: the Axis, that was formed by Germany, Italy and Japan, against the allies that was formed by Great Britain, France and Poland. The war broke out simultaneously in Europe, Africa, Asia and the Pacific and it developed in three phases. During the first one, Germany won various victories and conquered a great number of territories ,thanks to its strategy of blitzkrieg (rapid and successive air and land attacks on the enemy front) ,but in the Battle of Britain and in the invasion of the USSR ,which joined the Allies, it failed. At the same time, the italians were attacking the British protectorate of Egypt in order to gain control of the Mediterranean and  the Japanese were invading the French colonies in Indochina, beginning their expansion in the Pacific and attacking an American naval base, which carried the join of USA to the Allies. In the second phase, the Axis suffered its first major defeats like in the Battle of El Alamein, where they were expelled from egypt; in the Battle of the Atlantic, where the german submarines were destroyed;and in the battle of Stalingrad, were the germans were forced to surrender by the russians. Apart from that, during it, the war in Africa ended, Mussolini was dismissed and executed, the conference of Tehran took place, where the representatives of Britain (Churchill), USA (Roosevelt) and the USSR (Stalin) agreed to open a new front in France in order to liberate Western Europe ,and Hitler commited suicide. In the Third one, the effectiveness of aircraft carriers and American submarines in some battles like the Battle of the Coral Sea or of Midway, put an end to the Japanese control of the Pacific islands. Finally, the conflict ended when Harry Truman authorised the dropping of atomic bombs on the cities of Hiroshima and Nagasaki.
During the war, a wartime economy was established focused on supplying and supporting, so raw materials such as iron were overexploited for use in the industry, petroleum extraction increased to provide fuel, industries were focused on manufacturing military vehicles and scientific and technological research was focused on the developing of new weapons. Also, society was completely conditioned by the effects of the war, such as shortage of food or bombing, which led to the creation of rationing systems, to the evacuation of many civilians and to the change of their habits. In Germany, a part from that, a resistance movement ,which went against the nazis, and the collaborators ,who defended them,were created, and the number of exiles increased. During this war two important events took place: the Holocaust ,in which jews of all the countries occupied by the axis were executed by the nazis, and the creation of the UN to protect human rights and maintain peace, as result of the agreements between the Allies at the Conference of San Francisco and which replaced the League of nations.
This war had significant demographic consequences ,like the provoking of the greatest loss of human live in a war; economic ones ,like the destruction of areas of production and cities; political ones ,like the change in the leading systems of the Axis and the increase of power of USA and the USSR that became the world’s new superpowers and who divided the world in two main spheres of influence with different political systems ; and territorial ones, like the acquisition of the territories of the axis by the allies. Also, as a result of the active participation of the colonies in the war,  the decolonisation took place, in which all those colonies became independent and which caused the creation of new nations (with political instability, economic underdevelopment, neo-colonialism and high population growth)