What Were the Main Causes of World War II?

World War II era battle and bombing. Image credit: Ivan Cholakov/Shutterstock.com
  • World War II was the bloodiest conflict in human history, with up to 85 million people killed.
  • WWII pitted the Allies, led by the U.S., the U.K., France, and the Soviet Union against the Axis, led by Germany, Italy, and Japan.
  • Unprovoked attacks against the U.S. and the Soviet Union led them to enter the war.
  • WWII officially began on September 1, 1939 and ended on September 2, 1945.

World War II, which took place between September 1939 and September 1945, was the bloodiest conflict in human history. Though no exact casualty count for the war exists, it is thought that up to 85 million people were killed as a direct or indirect result of the conflict. The war involved all of the world’s great powers, and touched nearly every corner of the globe. On one side was the Axis, led by Germany, Italy, and Japan. On the other side were the Allies, led by the United States, the United Kingdom, France, and the Soviet Union. There were numerous reasons for the outbreak of the Second World War, from the megalomaniac ambitions of ruthless dictators, to the unprovoked attacks on countries that precipitated their entry into the war. Here are the main causes for the outbreak of WWII, humanity’s most destructive conflict.

The Rise Of Dictators And Their Imperialist Ambitions

Benito Mussolini with Hitler on 25 October 1936.

Before WWII began, two men took power in the countries of Germany and Italy. One was Benito Mussolini, who became Prime Minister of Italy in 1922. The other was Adolf Hitler, who became the Chancellor of Germany in 1933. In just a short time, the two men turned their countries into tyrannical dictatorships. Mussolini sought the resurrection of the Roman Empire, while Hitler sought the formation of a new German empire, the Third Reich, which he wanted to stretch from the Ural Mountains in the east, all the way to the west coast of Europe.

The Treaty Of Versailles And The Desire For Revenge

Treaty of Versailles. Image credit: Auckland War Memorial Museum/Public domain

Hitler was able to ascend to power in Germany in part because he was successful in tapping the anger of the German people in response to the terms of their surrender in the First World War. The Treaty of Versailles compelled Germany to give up large amounts of territory, and pay war reparations, which significantly damaged the German economy. Hitler promised his people revenge against the Allied powers, and the expansion of German power and territory.

The Great Depression

One of the most famous American photos of the 1930s, shows Florence Owens Thompson, mother of seven children, 32 years old, in Nipono, California, March 1936, looking for a job or social aid to support her family. Image credit: Dorothea Lange/Public domain

The economic ruin brought on by the Great Depression of the 1930s helped bring about the rise of many extremist movements in Europe, of which Nazism and fascism were just two. During the Depression, the Nazis and fascists did battle with the communists, but it was the former movements who emerged victorious in the end.

Pre-WWII Conquests

Italian invasion of Albania

Before WWII had officially begun, the powers that would merge to form the Axis had already launched campaigns of conquest. Shortly after Hitler came to power, he managed to seize control of Austria and part of what was then Czechoslovakia without any major combat operation. Italy had already conquered both Ethiopia and Albania, and Japan was expanding its imperial realm decades before WWII began, conquering the Korean peninsula, Taiwan, and the southern half of the Far East Soviet island of Sakhalin. In 1931, Japan began its attempt to conquer China by invading Manchuria. The fact that the Axis powers were able to expand their territory with little to no resistance from rest of the international community only emboldened them to make further conquests. 

The Failure Of Appeasement

Adolf Hitler at Prague Castle

One leader of the Western powers, British Prime Minister Neville Chamberlain, believed he could placate Adolf Hitler by appeasing him, thus allowing the Nazi dictator to take a small bit of territory in Czechoslovakia without provoking a war, and getting Hitler to promise that future territorial disputes would be resolved through non-violent means. But just a few months later, Hitler broke his promise and seized all of Czechoslovakia.

Unprovoked Attacks Against Uninvolved Powers

Pearl Harbor: Three stricken U.S. battleships on December 7, 1941. Image credit: Everett Collection/Shutterstock.com

The Nazi invasion of Poland, beginning on September 1, 1939, was the catalyst for the outbreak of WWII, when the Allied powers finally gave up on diplomacy and appeasement, and decided that only force could stop the Axis powers from making any new conquests. But even after the Nazis’ invasion of Poland, the war was still largely a European conflict. The Soviet Union and the United States had not yet begun fighting the Axis powers. Unprovoked attacks against them, however, changed that. In the summer of 1941, the Nazis invaded the Soviet Union, drawing the communist dictatorship into the conflict. Just six months later, another unprovoked attack led to the U.S. entering the war on the side of the Allies, when on December 7, 1941, Japanese forces bombed the U.S. Naval base at Pearl Harbor.


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