Causes of World War II

The causes of World War II help us learn about the events that led to one of the biggest wars in history. The war began when Nazi Germany invaded Poland on September 1, 1939. This attack caused Britain and France to declare war on Germany. But many things happened before this invasion that made such a big war more likely.

One major reason was the anger in Germany after World War I. The Treaty of Versailles in 1919 made Germany pay large amounts of money and limited its military. Many Germans felt this was unfair. During the difficult economic times of the Great Depression in the 1930s, people in many countries lost trust in their governments. In Germany, Adolf Hitler and his Nazi Party took control in 1933. Hitler wanted Germany to become strong again.

Other countries also acted in ways that made tension grow. Fascist Italy attacked Ethiopia, and Imperial Japan fought against China. At first, larger powers like Britain and France tried to avoid war by allowing some of Germany’s demands, a policy called appeasement. But in 1938, Germany took part of Czechoslovakia anyway. By 1939, it became clear that peace could not be kept, and the war began.

Ultimate causes

World War II began because of things that happened after World War I. When World War I ended in 1918, many countries in Europe were having a hard time. France wanted to make sure Germany could not attack again, so they made Germany agree to strict rules called the Treaty of Versailles. These rules made many Germans very unhappy.

The Treaty of Versailles made Germany pay a lot of money and gave away some of its land. It also limited Germany’s military, which many Germans felt was unfair. This anger helped lead to World War II.

Other countries also had problems. Japan wanted more land and resources, so they took over parts of China. Italy, led by Benito Mussolini, wanted to build a big empire too. Germany, under Adolf Hitler, also wanted to grow more powerful. All these actions caused tension. When Germany invaded Poland in 1939, Britain and France declared war, and that started World War II.

Proximate causes

Nazi dictatorship

Hitler and his Nazis took control of Germany in 1933–34, making it a dictatorship. They were unhappy about the Treaty of Versailles and how Jews were treated. They spent a lot on the military to help with jobs.

Hitler made demands that seemed fair but could lead to war if not met. He accepted some changes but then made new demands. His aggressive actions helped Germany leave the League of Nations, ignore the Versailles Treaty, build up its military, and take over some lands. Hitler made agreements but then broke them.

Remilitarization of the Rhineland

Germany sent troops into the Rhineland on March 7, 1936, breaking the Treaty of Versailles and other agreements. France and Britain were not ready to fight, so nothing happened.

Italian invasion of Abyssinia

The Italian leader Benito Mussolini wanted to grow the Italian Empire by invading Ethiopia in Africa. The League of Nations said Italy was the attacker and put some restrictions on them, but these did not work. Italy took over Ethiopia and joined it with Eritrea and Somaliland into Italian East Africa. Ethiopia's leader Haile Selassie spoke against Italy at the League of Nations.

Spanish Civil War

From 1936 to 1939, Germany and Italy helped the Nationalists led by General Francisco Franco in Spain, while the Soviet Union helped the Spanish Republic. Both sides tried out new weapons and battle methods. The Nationalists won in 1939. Spain did not join World War II but sent some volunteers to help Germany.

Second Sino-Japanese War

In 1931, Japan used China's weakness to create a puppet state in Manchuria. In 1937, the Marco Polo Bridge Incident started the Second Sino-Japanese War. Japan bombed cities and captured the Chinese capital, Nanjing. The war used up many Chinese soldiers, and Japan set up puppet states for support.

Anschluss

The Anschluss was when Germany took over Austria in 1938. The idea of a Greater Germany including all ethnic Germans was popular in both places. Austria's government resisted but had no help outside and finally agreed to Hitler's demands. There was no fighting, and Austria became part of Germany.

Munich Agreement

The Sudetenland was a part of Czechoslovakia with many ethnic Germans. Hitler wanted to include it in Germany, with support from German groups there. In the Munich Agreement of 1938, Britain, France, and Italy let Germany take the Sudetenland, hoping it would be Hitler's last request. Czechoslovakia was not invited. Germany took the area without fighting.

In March 1939, Germany broke the Munich Agreement and invaded Prague. Slovakia became independent, and Czechoslovakia no longer existed. This ended the plan by France and Britain to avoid war.

Italian invasion of Albania

After Germany took over Czechoslovakia, Mussolini wanted Italy to not be less important in the Axis. Italy invaded Albania in April 1939 after Albania refused an ultimatum. Albanian forces did not fight much, and Italy took over the country.

Soviet–Japanese border war

In 1939, Japan attacked into Mongolia from Manchuria but was defeated by Soviet forces under General Georgy Zhukov. After this battle, the Soviet Union and Japan were peaceful until 1945. Japan focused on the south, leading to fights with the United States. The Soviet Union kept soldiers on its border with Japan.

Danzig crisis

After Germany broke the Munich Agreement, Britain and France decided to stop any more German expansion by using force. The next target was Poland, especially the port of Danzig, which was a free city under Polish control. In 1939, Hitler asked for changes to Danzig's status, but Poland said no. In August 1939, Hitler gave Poland an ultimatum about Danzig.

Polish alliance with the Entente

Poland had military agreements with France and Britain. These agreements were to help each other if Nazi Germany invaded.

Interwar period

During the Polish–Soviet War in 1920, France helped Poland with military support. In February 1921, leaders from Poland and France talked about three important agreements: political, military, and economic.

The political agreement was signed on February 19, 1921. It promised to work together on foreign policy, support each other economically, and help if one was attacked without warning. The military agreement, signed two days later, focused on protecting against threats from Germany and the Soviet Union. If Poland was attacked, France would keep communication lines open and watch Germany but did not have to send soldiers or declare war. These agreements became official when the economic pact was ratified on August 2, 1923.

Later, in 1925, the agreements were added to the system of mutual pacts of the League of Nations through the Franco–Polish Warrant Agreement. These alliances aimed to stop Germany from using force to change borders after World War I. However, the alliances did not work as well as hoped because of various issues, such as Czechoslovakia not fully supporting Poland and France not investing enough in its allies.

1939

Molotov–Ribbentrop Pact

Further information: Molotov–Ribbentrop Pact, Soviet invasion of Poland, Occupation of the Baltic states, Soviet occupation of Bessarabia and Northern Bukovina, and Winter War

The Molotov–Ribbentrop Pact was a treaty between Germany and the Soviet Union, signed on August 23, 1939. While it was called a "non-aggression treaty," it had a secret part that divided several countries, including Poland, into areas of control for each country.

When Germany invaded Poland on September 1, 1939, Britain and France declared war on Germany. Soon after, on September 17, the Soviet Union also invaded Poland from the east, saying they needed to protect people living there. These attacks ended the existence of the Polish state.

Declarations of war

Invasion of Poland

Further information: Invasion of Poland

Between 1919 and 1939, Poland tried to stay balanced between the Soviet Union and Nazi Germany. In 1939, Germany asked Poland to join them, but Poland said no, wanting to keep its independence. Hitler wanted to invade Poland to get more land. Britain and France told Hitler that if he invaded Poland, it would mean war. However, the Soviet Union agreed to work with Germany in secret in August 1939, which helped Germany invade Poland. Germany invaded Poland on September 1, 1939. Because of this, Britain and France declared war on Germany on September 3, 1939. Neither country sent much help to Poland.

On September 17, 1939, the Soviet Union entered Poland from the east. Polish forces stopped defending and moved to nearby countries. The last large Polish unit surrendered on October 6, 1939.

Invasion of the Soviet Union

Further information: Operation Barbarossa and Soviet offensive plans controversy

Germany attacked the Soviet Union in June 1941. Hitler thought they could defeat the Soviet Union quickly and hoped this would bring Britain to the negotiating table.

Attacks on Pearl Harbor, the Philippines, British Malaya, Singapore and Hong Kong

Further information: Prelude to the attack on Pearl Harbor

The US government supported China and did not agree with Japan’s actions there. The US limited trade with Japan in 1940 and stopped all oil shipments in 1941. Japan invaded parts of French Indochina to stop trade with China. In July 1941, Japan took control of the rest of French Indochina.

On December 7, 1941, Japan attacked Pearl Harbor to damage the main American battle fleet. At the same time, Japanese forces attacked the Philippines, and the British Empire in Malaya, Singapore, and Hong Kong. The next day, Japan officially declared war on the United States and the British Empire. Canada declared war on Japan on the evening of December 7, and Britain declared war on Japan on December 8. The United States declared war on Japan on December 8 as well.

Four days later, on December 11, 1941, Germany and Italy declared war on the United States, which brought the US into the European war as well.