Humanities › History & Culture When Did World War II Start? Share Flipboard Email Print Hulton Archive/Stringer/Archive Photos History & Culture Military History World War II Battles & Wars Key Figures Arms & Weapons Naval Battles & Warships Aerial Battles & Aircraft Civil War French Revolution Vietnam War World War I American History African American History African History Ancient History and Culture Asian History European History Genealogy Inventions Latin American History Medieval & Renaissance History The 20th Century Women's History View More By Jennifer Rosenberg History Expert B.A., History, University of California at Davis Jennifer Rosenberg is a historian and writer who specializes in 20th-century history. our editorial process Jennifer Rosenberg Updated March 23, 2020 After the horrors of World War I, no one wanted war. However, when Germany attacked Poland on September 1, 1939, other European countries felt they had to act. The result was six long years of World War II. Learn more about what led to Germany's aggression and how other countries reacted. Hitler's Ambitions Adolf Hitler wanted more land, to expand Germany according to the Nazi policy of "lebensraum"—a German word meaning roughly "living space," and lebensraum served as Hitler's justification for expanding his empire eastward. Hitler used the harsh limitations that had been set against Germany after World War I in the Versailles Treaty as a pretext for Germany's right to acquire land where German-speaking people lived. Germany successfully used this reasoning to envelop two entire countries without starting a war. Austria: On March 13, 1938, Germany took over Austria (termed the Anschluss)—a contingency specifically disallowed in the Versailles Treaty.Czechoslovakia: At the Munich Conference on September 28–29, 1938, the French and the British handed Germany a large portion of Czechoslovakia. Hitler then took the rest of Czechoslovakia by March 1939. Many people have wondered why Germany was allowed to take over both Austria and Czechoslovakia without a fight. The simple reason is that Great Britain and France did not want to repeat the bloodshed of World War I. Britain and France believed, wrongly as it turned out, they could avoid another world war by appeasing Hitler with a few concessions (such as Austria and Czechoslovakia). At this time, Great Britain and France did not understand that Hitler's hunger for land acquisition was much, much more ambitious than any one country could slake. The Excuse: Operation Himmler After having gained both Austria and Czechoslovakia, Hitler was confident that he could again move east, this time acquiring Poland without having to fight Britain or France. (To eliminate the possibility of the Soviet Union fighting if Poland were attacked, Hitler made a pact with the Soviet Union—the Nazi-Soviet Non-Aggression Pact.) So that Germany did not officially seem the aggressor (which it was), Hitler needed an excuse for attacking Poland. It was Heinrich Himmler who came up with the idea; thus the plan was code-named Operation Himmler. On the night of August 31, 1939, Nazis took an unknown prisoner from one of their concentration camps, dressed him in a Polish uniform, took him to the town of Gleiwitz (on the border of Poland and Germany), and then shot him. The staged scene with the dead prisoner dressed in a Polish uniform was supposed to appear as a Polish attack against a German radio station. Hitler used this staged attack as the excuse to invade Poland. Blitzkrieg At 4:45 on the morning of September 1, 1939 (the morning following the staged attack), German troops entered Poland. The sudden, immense attack by the Germans was called a Blitzkrieg ("lightning war"). The German air attack hit so quickly that most of Poland's air force was destroyed while still on the ground. To hinder Polish mobilization, the Germans bombed bridges and roads. Groups of marching soldiers were machine-gunned from the air. But the Germans did not just aim for soldiers; they also shot at civilians. Groups of fleeing civilians often found themselves under attack. The more confusion and chaos the Germans could create, the slower Poland could mobilize its forces. Using 62 divisions, six of which were armored and ten mechanized, the Germans invaded Poland by land. Poland was not defenseless, but they could not compete with Germany's motorized army. With only 40 divisions, none of which were armored, and with nearly their entire air force demolished, the Poles were at a severe disadvantage. The Polish cavalry was no match for German tanks. Declarations of War On September 1, 1939, the beginning of the German attack, Great Britain, and France sent Adolf Hitler an ultimatum: Germany must either withdraw his forces from Poland, or Great Britain and France would go to war against him. On September 3, with Germany's forces penetrating deeper into Poland, Great Britain and France both declared war on Germany. World War II had begun.