Causes of World War II

The causes of World War II are important to understand because they 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 and wanted to change things. 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 and pursued aggressive policies.

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 largely because of events that happened after World War I. When World War I ended in 1918, many countries, especially in Europe, were struggling. France wanted to make sure Germany could not attack again, so they made Germany agree to harsh rules called the Treaty of Versailles. These rules made Germans very unhappy.

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

Other countries also had problems. Japan wanted more land and resources, so they began taking over parts of China. Italy, led by Benito Mussolini, also wanted to build a large empire. Meanwhile, Germany, under Adolf Hitler, wanted to expand its power too. All these actions created tension, and eventually, when Germany invaded Poland in 1939, Britain and France declared war, starting World War II.

Proximate causes

Nazi dictatorship

Hitler and his Nazis took control of Germany in 1933–34, turning it into a dictatorship. They were very unhappy about the Treaty of Versailles and the way Jews were treated. They used heavy military spending to solve unemployment problems.

Hitler made demands that seemed reasonable but threatened war if they were not met. He accepted concessions and then moved on to new demands. His aggressive strategy worked as Germany pulled out of the League of Nations, rejected the Versailles Treaty, began to rearm, and made several territorial gains. Hitler made agreements and then broke them.

Remilitarization of the Rhineland

Germany moved troops into the Rhineland on March 7, 1936, breaking the Treaty of Versailles and other agreements. Neither France nor Britain was ready to fight to stop this, so there were no consequences.

Italian invasion of Abyssinia

The Italian dictator Benito Mussolini tried to expand the Italian Empire by invading Ethiopia in Africa. The League of Nations declared Italy the aggressor and imposed sanctions, but they were ineffective. Italy annexed Ethiopia and merged it with Eritrea and Somaliland into Italian East Africa. Ethiopian Emperor Haile Selassie spoke out against Italy's actions at the League of Nations.

Spanish Civil War

Between 1936 and 1939, Germany and Italy supported the Nationalists led by General Francisco Franco in Spain, while the Soviet Union supported the Spanish Republic. Both sides tested new weapons and tactics. The Nationalists eventually won in 1939. Spain stayed neutral during World War II but sent a volunteer unit to help Germany.

Second Sino-Japanese War

In 1931, Japan took advantage of China's weakness and set up a puppet state in Manchuria. In 1937, the Marco Polo Bridge Incident triggered the Second Sino-Japanese War. Japan bombed many cities and captured the Chinese capital of Nanjing. The war tied down many Chinese soldiers, and Japan set up puppet states to gain support.

Anschluss

The Anschluss was the annexation of Austria into Germany in 1938. The idea of creating a Greater Germany to include all ethnic Germans was popular in both countries. The Austrian government resisted but had no outside support and finally gave in to Hitler's demands. No fighting occurred, and Austria was fully absorbed into Germany.

Munich Agreement

The Sudetenland was a region in Czechoslovakia with many ethnic Germans. Hitler pressed for its incorporation into Germany, supported by German separatist groups. In the Munich Agreement of 1938, Britain, France, and Italy allowed Germany to take the Sudetenland in the hope it would be Hitler's last demand. Czechoslovakia was not allowed to participate. Germany took the region unopposed.

In March 1939, Germany broke the Munich Agreement and invaded Prague. With Slovakia declaring independence, Czechoslovakia disappeared as a country. This ended the French and British policy of appeasement.

Italian invasion of Albania

After Germany occupied Czechoslovakia, Mussolini wanted to avoid Italy becoming a lesser member of the Axis. Italy invaded Albania in April 1939 after Albania refused to accept an ultimatum. Albanian forces offered minimal resistance, and Italy occupied the country.

Soviet–Japanese border war

In 1939, Japan attacked into Mongolia from Manchuria but was defeated by Soviet units under General Georgy Zhukov. After this battle, the Soviet Union and Japan were at peace until 1945. Japan turned its focus south, leading to conflict with the United States. The Soviet Union kept troops on its border with Japan.

Danzig crisis

After Germany broke the Munich Agreement, Britain and France decided to oppose any further German expansion by force. The next target was Poland, especially the port of Danzig, which had been made a free city-state under Polish influence. In 1939, Hitler demanded changes to Danzig's status, but Poland refused. In August 1939, Hitler issued an ultimatum to Poland on Danzig's status.

Polish alliance with the Entente

Poland had military alliances with France and Britain. These alliances were meant to provide mutual assistance in case of an invasion from Nazi Germany.

Interwar period

During the Polish–Soviet War in 1920, France helped Poland by sending military support. In February 1921, leaders from Poland and France discussed 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 prevent 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

See also: Anglo-Polish alliance

In 1939, new agreements began to form to help protect countries from attacks. On May 19, 1939, an important military agreement was signed in Paris between Poland and France. This agreement promised that both countries would help each other if Germany attacked. However, when Germany did attack Poland, France's help was very small.

Britain and France had promised to support Poland if Germany attacked. They made this promise official in August 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 early 1939, Germany asked Poland to join as a follower of Germany, but Poland refused, fearing loss of independence. Hitler told his generals that he wanted to invade Poland to gain more land in the East. Britain and France warned Hitler that invading Poland would mean war. However, the Soviet Union allied with Germany through the secret Molotov–Ribbentrop Pact in August 1939, which helped Germany invade Poland. Germany invaded Poland on September 1, 1939. Britain and France declared war on Germany on September 3, 1939, because of this invasion. Neither country provided much military help to Poland.

On September 17, 1939, the Soviet Union entered Poland from the east. The Polish forces decided to stop 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 the Soviet Union could be defeated 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 opposed Japan’s actions in China. The US began limiting trade with Japan in July 1940, and stopped all oil shipments in 1941. Japan invaded parts of French Indochina to stop trade with China. In July 1941, Japan occupied the rest of French Indochina.

On December 7, 1941, Japan attacked Pearl Harbor to destroy 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.