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Learn Index · History and Change Grade 9–10

World War I and World War II

World War I (1914–1918) grew from alliances, militarism, imperialism and nationalism, and World War II (1939–1945) followed the Great Depression, aggression by the Axis powers and the Holocaust, and the wars reshaped the world.

Illustrated topic card: World War I and World War II — Two world wars in 30 years reshaped borders, governments and technology, and led to new efforts for lasting peace.
Illustrated topic card: World War I and World War II — Two world wars in 30 years reshaped borders, governments and technology, and led to new efforts for lasting peace.

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The First World War (1914–1918)

Causes: M-A-I-N

Historians often remember the long-term causes with the acronym MAIN:

Cause Explanation
Militarism European powers built huge armies and navies, such as the British–German naval race
Alliances Europe split into two alliances: the Triple Entente (France, Russia, Britain) and the Triple Alliance (Germany, Austria-Hungary, Italy)
Imperialism competition for colonies and resources, especially in Africa and Asia
Nationalism intense pride and rivalry; Slavic nationalism in the Balkans

The spark: On June 28, 1914, Archduke Franz Ferdinand of Austria-Hungary was assassinated in Sarajevo by Gavrilo Princip, a Bosnian Serb nationalist. Austria-Hungary declared war on Serbia, and the alliance system pulled the major powers into war within weeks.

The war

  • Western Front: trench warfare from Belgium to Switzerland; battles like Verdun and the Somme (1916) killed hundreds of thousands for little ground.
  • New technology: machine guns, poison gas, tanks, airplanes and submarines made the war deadlier.
  • Total war: governments directed whole economies to the war, used propaganda and drafted soldiers.
  • 1917: the United States entered the war; Russia left after its revolution.
  • November 11, 1918: an armistice ended the fighting.

About 9 to 10 million soldiers and millions of civilians died.

The peace

The Treaty of Versailles (1919) blamed Germany for the war, took territory, limited its army and demanded large payments, called reparations. Empires broke apart: Austria-Hungary, the Ottoman Empire and the Russian Empire. The League of Nations was created to keep peace, but the U.S. never joined.

Between the wars

  • The Great Depression (from 1929) caused mass unemployment worldwide.
  • Anger over Versailles and economic hardship helped extreme movements rise.
  • Fascism in Italy under Benito Mussolini; Nazism in Germany under Adolf Hitler (in power from 1933); militarism in Japan.
  • Germany rebuilt its military, took over Austria (1938) and Czechoslovakia (1938–39).
  • Britain and France followed appeasement, giving in to demands to avoid war, most famously at the Munich Agreement (1938).

The Second World War (1939–1945)

The war begins

On September 1, 1939, Germany invaded Poland. Britain and France declared war on September 3.

Allies Axis
Britain, France, Canada and the Commonwealth; later the Soviet Union (1941), United States (1941), China and others Germany, Italy, Japan

Key events

Year Event
1940 Germany conquers France; the Battle of Britain
1941 Germany invades the Soviet Union; Japan attacks Pearl Harbor and the U.S. enters the war
1942–43 Battle of Stalingrad, a major turning point; battles at Midway and El Alamein
June 6, 1944 D-Day: Allied landings in Normandy
May 8, 1945 Victory in Europe (VE Day)
August 1945 atomic bombs dropped on Hiroshima and Nagasaki
September 2, 1945 Japan formally surrenders

The war killed an estimated 70 to 85 million people, most of them civilians.

The Holocaust

The Holocaust was the Nazi regime’s systematic, state-sponsored murder of six million Jews, about two-thirds of Europe’s Jewish population. The Nazis also murdered Roma and Sinti, disabled people, Polish and Soviet civilians, gay men, political opponents and others. Victims were killed in mass shootings, ghettos and death camps such as Auschwitz-Birkenau. The Holocaust shows where hatred, propaganda and dehumanization can lead, and why people must speak up against antisemitism and all forms of hate.

Consequences of the World Wars

  • The United Nations (1945) was founded to prevent future wars.
  • The Universal Declaration of Human Rights (1948)
  • Trials at Nuremberg and Tokyo held leaders responsible for war crimes and crimes against humanity
  • The United States and the Soviet Union became superpowers, leading to the Cold War
  • Decolonization: many colonies in Asia and Africa gained independence
  • Europe began working toward economic cooperation, which later became the European Union

Try it yourself

  1. Rank the MAIN causes of WWI from most to least important and defend your ranking.
  2. Compare the causes of WWI and WWII. What was similar? What was different?
  3. Explain how the Treaty of Versailles contributed to WWII.
  4. Research a survivor’s testimony from the Holocaust and reflect on its lessons.

Quick recap

  • Militarism, alliances, imperialism and nationalism caused WWI; an assassination was the spark.
  • WWI was a total war with new technology and trench warfare.
  • The harsh peace, the Depression and fascism helped lead to WWII.
  • WWII was the deadliest war in history, and included the Holocaust.
  • The wars led to the UN, human rights declarations, the Cold War and decolonization.