Causes of WWII

Discussing plans for Operation Barbarossa, the upcoming invasion of the Soviet Union during the Second World War, Nazi German leader Adolf Hitler informed the chiefs of staff of his army in March 1941:

What led to the bloodiest global conflict in history, World War II? Were the causes simple or complex? Could this war have been prevented? Historians underscore several long-term and short-term contributors to this event.

Nazi German soldiers in front of burning houses and a church outside Leningrad (St. Petersburg), Soviet Union, autumn 1941. Source: National Digital Archives of Poland, Wikipedia Commons (public domain).

Causes of WWII in Europe and Asia

There were several long-term and short-term causes of the Second World War. The long-term causes include:

• The Treaty of Versailles (1919).
• The Great Depression (1929).
• German and Japanese militarism.
• German Nazism and Japanese imperialism.
• The failure of peace initiatives (the Kellogg-Briand Pact and the League of Nations).
• The failure of non-aggression pacts between several countries and Germany and appeasement through the Munich Agreement (1938).

The short-term lead-up to the war comprised several events:

• Japan invaded China’s Manchuria in 1931 (the Mukden Incident).
• Italy under fascist leader Benito Mussolini invaded Ethiopia in 1935 (the Abyssinian Crisis).
• Full-scale war between Japan and China: the Second Sino-Japanese War began in 1937.
• Germany acquired Austria in 1938.
• Germany annexed Sudetenland in Czechoslovakia in 1938.
• Germany invaded Poland in 1939 starting the Second World War.

Long-Term Causes of WWII

There are up to half a dozen long-term causes of the bloodiest military conflict in history.

The Treaty of Versailles (1919)

The Treaty of Versailles was an important aspect of the Paris Peace Conference (1919-1920) which concluded WWI. This agreement dictated the terms of the postwar settlement.

Treaty of Versailles cover, ca. June 28th, 1919. Source: Auckland War Memorial Museum, Wikipedia Commons (public domain).

The scar WWI left on Europe was deep and bloody, the resentment animated the term of surrender and reconciliation. The treaty was between the victors of this conflict, Britain, the United States, Japan, and France, and the vanquished, Germany. Neither Germany nor its wartime allies Hungary and Austria–the Central Powers–were allowed to define the contents of the treaty. The victors punished Germany by blaming it for the war. As a result, Germany, known as the Weimar Republic from 1918-1933, was ordered to:

• reduce its weapons stockpiles and the size of its armed forces in a process of demilitarization;
• pay reparations to the affected countries;
• give up several territories to France, Belgium, Poland, and Czechoslovakia, as well as its colonies abroad.

In addition, Austria lost such territories as Sudetenland to Czechoslovakia through another postwar agreement, the Treaty of Saint Germain (1919), which became an important lead-up to the Second World War.

The Inter-Allied Military Commission of Control oversaw Germany’s adherence to the terms of demilitarization, for instance, limiting its army to 100,000 men, and reducing weapons ownership and the import and export of materiel.

Furthermore, border disputes persisted. According to Germany, millions of Germans were now stranded in foreign countries because of the Versailles Treaty. The Pact of Locarno (1925) was supposed to confirm the German border with France and Belgium, respectively, but it did not help in the long term.

Economic Causes of WWII

The Weimar Republic was in a terrible economic state and experienced hyperinflation of its currency in the early 1920s. The American-led Dawes and Young Plans in 1924 and 1929, respectively, were meant to relieve some of the economic pain through loans and other financial mechanisms.

German Railways Banknote, 5 billion marks during the hyperinflation period in 1923. Source: Wikipedia Commons (public domain).

The Great Depression arrived with the U.S. stock market crash of 1929. It led to joblessness, homelessness, and hunger for the public, along with bank failures and a significant decline in the gross national product.

Germany began to slowly recover by the time Adolf Hitler came to power in 1933, and the country became known as the Third Reich. However, the populist support of the Nazi (National Socialist) Party came from the preceding economic conditions.

Adolf Hitler, 1936. Source: Bundesarchiv, Bild 146-1990-048-29A / CC-BY-SA 3.0, Wikipedia Commons.

Failure of the League of Nations

In addition to the Treaty of Versailles, the League of Nations was the second important result of the Paris Peace Conference. Representatives of more than 30 countries worked to establish the League—an international organization meant to foster global peace.

In decade later, 15 countries, followed by dozens of others, signed the Kellogg-Briand Pact (1928):

• The U.S.
• Germany
• Britain
• France
• Japan

This agreement also sought to prevent war. However, the Kellogg-Briand Pact lacked enforcement mechanisms. In 1931, Japan attacked China's Manchuria. The League of Nations failed to adequately punish Japan, and the Kellogg-Briand Pact was ambiguous. Several other incidents, such as Italy's invasion of Ethiopia (1935), discredited the international legal system in the 1930s and set the world on a path to war.

Failure of International Agreements

There were many agreements signed between different countries in the interwar period. Some agreements reinforced the Treaty of Versailles like the Locarno Pact. Others sought to foster peace in general, such as the Kellogg-Briand Pact. Other agreements with Germany, like the non-aggression pacts, sought to prevent war between the given signatories, such as the Molotov-Ribbentrop Pact between the Soviet Union and Germany. Finally, the ineffective appeasement of the Munich Agreement ceded territories to Hitler—Sudetenland in Czechoslovakia—to prevent greater war.

Munich Agreement signatories, (L-R) Britain’s Chamberlain, France’s Daladier, Germany’s Hitler, Italy’s Mussolini and Ciano, September 1938. Source: Bundesarchiv, Bild 183-R69173 / CC-BY-SA 3.0, Wikipedia Commons.

German Nazism, Japanese Imperialism, and Militarism

In Europe, Nazi ideology under Adolf Hitler featured a racialist, supremacist hierarchy, in which ethnic Germans were at the top, and others, like Jews and Slavs, were considered inferior (Untermenschen). The Nazis also subscribed to the concept of Lebensraum, "living space." They believed that they were entitled to acquire Slavic lands for ethnic Germans. This idea was one of the motivations for the invasion of the Soviet Union in June 1941.

Emperor Hirohito channeling militarist aesthetics on his favorite white horse: Shirayuki (White Snow), 1935. Source: Osaka Asahi Shimbun, Wikipedia Commons (public domain).

In Asia, the Japanese empire under Emperor Hirohito invaded other countries from 1931-1945, having already annexed Korea in 1910. Japan invaded China's Manchuria in 1931, the rest of China in 1937, and other countries in Southeast Asia, such as Vietnam during World War II. Japan called its empire the Greater East Asia Co-Prosperity Sphere. In reality, Japan extracted the resources it needed from its colonies.

Short-Term Causes of WWII: German Invasion of Poland

Poland and Hungary invaded Czechoslovakia after Germany annexed Sudetenland in that country in October 1938. However, these events did not prevent Poland from being invaded by Germany on September 1, 1939. That date marked the start of the Second World War.

Two days later, both France and Britain declared war on Germany. On September 17, the Soviet Union also entered Poland in an attempt to push the conflict away from the Soviet borders. This attempt to avoid war also failed when Germany invaded the Soviet Union on June 22, 1941.

By the start of World War II in Europe, the Second Sino-Japanese War had been raging in Asia since 1937. The two conflicts turned into one with Japan's strike on America's Pearl Harbor on December 7, 1941, making the war truly global.

Consequences of WWII

There were many important consequences of the Second World War, including:

• The Soviet Union and the United States became superpowers after it ended, in 1945. They were no longer Allies but opponents in the Cold War (1945-1991), which split the world into two competing blocs.
• The United Nations replaced the League of Nations with four Allies (Soviet Union, United States, Britain, and China), and France, as the permanent members of the Security Council.
• The United States used the atomic bomb for the first time in history against Japan's Hiroshima and Nagasaki. From then on, a nuclear arms race began.
• The decolonization process continued in Asia and Africa. Many countries became independent. However, in some cases, this process was accompanied by military conflicts, such as the Vietnam War.