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The causes of World War II, a global war from 1939 to 1945 that was the deadliest conflict in human history, have been given considerable attention by historians from many countries who studied and understood them. The immediate precipitating event was the invasion of Poland by Nazi Germany on September 1, 1939, and the subsequent declarations of war on Germany made by Britain and France, but many other prior events have been suggested as ultimate causes. Primary themes in historical analysis of the war's origins include the political takeover of Germany in 1933 by Adolf Hitler and the Nazi Party; Japanese militarism against China, which led to the Second Sino-Japanese War; Italian aggression against Ethiopia, which led to the Second Italo-Ethiopian War and Germany's initial success in negotiating the Molotov–Ribbentrop Pact with the Soviet Union to divide the territorial control of Eastern Europe between them.
During the interwar period, deep anger arose in the Weimar Republic on the conditions of the 1919 Treaty of Versailles, which punished Germany for its role in World War I with severe conditions and heavy financial reparations to prevent it from ever becoming a military power again. That provoked strong currents of revanchism in German politics, with complaints primarily focused on the demilitarisation of the Rhineland, the prohibition of German unification with Austria and the loss of some German-speaking territories and overseas colonies.
During the worldwide economic crisis of the Great Depression in the 1930s, many people lost faith in democracy and countries across the world turned to authoritarian regimes.[1] In Germany, resentment and hatred of other countries was intensified by the instability of the German political system, as many activists rejected the Weimar Republic's legitimacy. The most extreme political aspirant to emerge from that situation was Adolf Hitler, the leader of the Nazi Party. The Nazis took totalitarian power in Germany from 1933 and demanded the undoing of the Versailles provisions. Their ambitious and aggressive domestic and foreign policies reflected their ideologies of Antisemitism, unification of all Germans, the acquisition of "living space" (Lebensraum) for agrarian settlers, the elimination of Bolshevism and the hegemony of an "Aryan"/"Nordic" master race over "subhumans" (Untermenschen) such as Jews and Slavs. Other factors leading to the war included the aggression by Fascist Italy against Ethiopia and by Imperial Japan against China.
World War II was the biggest and deadliest war in history, involving more than 30 countries. Sparked by the 1939 Nazi invasion of Poland, the war dragged on for six bloody years until the Allies defeated Nazi Germany and Japan in 1945.
World War II, also called Second World War, conflict that involved virtually every part of the world during the years 1939–45. The principal belligerents were the Axis powers—Germany, Italy, and Japan—and the Allies—France, Great Britain, the United States, the Soviet Union, and, to a lesser extent, China. The war was in many respects a continuation, after an uneasy 20-year hiatus, of the disputes left unsettled by World War I. The 40,000,000–50,000,000 deaths incurred in World War II make it the bloodiest conflict, as well as the largest war, in history.
By the early part of 1939 the German dictator Adolf Hitler had become determined to invade and occupy Poland. Poland, for its part, had guarantees of French and British military support should it be attacked by Germany. Hitler intended to invade Poland anyway, but first he had to neutralize the possibility that the Soviet Union would resist the invasion of its western neighbour. Secret negotiations led on August 23–24 to the signing of the German-Soviet Nonaggression Pact in Moscow. In a secret protocol of this pact, the Germans and the Soviets agreed that Poland should be divided between them, with the western third of the country going to Germany and the eastern two-thirds being taken over by the U.S.S.R.
Having achieved this cynical agreement, the other provisions of which stupefied Europe even without divulgence of the secret protocol, Hitler thought that Germany could attack Poland with no danger of Soviet or British intervention and gave orders for the invasion to start on August 26. News of the signing, on August 25, of a formal treaty of mutual assistance between Great Britain and Poland (to supersede a previous though temporary agreement) caused him to postpone the start of hostilities for a few days. He was still determined, however, to ignore the diplomatic efforts of the western powers to restrain him. Finally, at 12:40 PM on August 31, 1939, Hitler ordered hostilities against Poland to start at 4:45 the next morning. The invasion began as ordered. In response, Great Britain and France declared war on Germany on September 3, at 11:00 AM and at 5:00 PM, respectively. World War II had begun.
In September 1939 the Allies, namely Great Britain, France, and Poland, were together superior in industrial resources, population, and military manpower, but the German Army, or Wehrmacht, because of its armament, training, doctrine, discipline, and fighting spirit, was the most efficient and effective fighting force for its size in the world. The index of military strength in September 1939 was the number of divisions that each nation could mobilize. Against Germany’s 100 infantry divisions and six armoured divisions, France had 90 infantry divisions in metropolitan France, Great Britain had 10 infantry divisions, and Poland had 30 infantry divisions, 12 cavalry brigades, and one armoured brigade (Poland had also 30 reserve infantry divisions, but these could not be mobilized quickly). A division contained from 12,000 to 25,000 men.
It was the qualitative superiority of the German infantry divisions and the number of their armoured divisions that made the difference in 1939. The firepower of a German infantry division far exceeded that of a French, British, or Polish division; the standard German division included 442 machine guns, 135 mortars, 72 antitank guns, and 24 howitzers. Allied divisions had a firepower only slightly greater than that of World War I. Germany had six armoured divisions in September 1939; the Allies, though they had a large number of tanks, had no armoured divisions at that time.
The six armoured, or panzer, divisions of the Wehrmacht comprised some 2,400 tanks. And though Germany would subsequently expand its tank forces during the first years of the war, it was not the number of tanks that Germany had (the Allies had almost as many in September 1939) but the fact of their being organized into divisions and operated as such that was to prove decisive. In accordance with the doctrines of General Heinz Guderian, the German tanks were used in massed formations in conjunction with motorized artillery to punch holes in the enemy line and to isolate segments of the enemy, which were then surrounded and captured by motorized German infantry divisions while the tanks ranged forward to repeat the process: deep drives into enemy territory by panzer divisions were thus followed by mechanized infantry and foot soldiers. These tactics were supported by dive bombers that attacked and disrupted the enemy’s supply and communications lines and spread panic and confusion in its rear, thus further paralyzing its defensive capabilities. Mechanization was the key to the German blitzkrieg, or “lightning war,” so named because of the unprecedented speed and mobility that were its salient characteristics. Tested and well-trained in maneuvers, the German panzer divisions constituted a force with no equal in Europe.
The German Air Force, or Luftwaffe, was also the best force of its kind in 1939. It was a ground-cooperation force designed to support the Army, but its planes were superior to nearly all Allied types. In the rearmament period from 1935 to 1939 the production of German combat aircraft steadily mounted.