What Was World War I? Causes, Phases and Consequences

Soldiers of A Company, 11th Cheshire Regiment, in a captured German trench near Ovillers-la-Boisselle during the Battle of the Somme, July 1916. Photograph by John Warwick Brooke, Imperial War Museums; image record and public-domain information.

World War I was a global war fought from 1914 to 1918. The assassination of Archduke Franz Ferdinand helped trigger the July Crisis, but the conflict grew out of longer rivalries, nationalism, military planning and alliance politics. It ended with an armistice on November 11, 1918; the peace settlements followed in 1919 and after.

What was World War I?

World War I, also called the Great War, began in Europe and spread across several continents, oceans and colonial territories. The main Central Powers were Germany, Austria-Hungary and the Ottoman Empire. The Allied side included France, Britain and Russia at the start; other countries joined over time. Italy entered on the Allied side in 1915, and the United States declared war on Germany in April 1917.

Calling it a world war does not mean every country fought. It reflects the reach of empires, overseas territories, colonial troops, global shipping and combat beyond Europe, including in Africa, the Middle East and East Asia.

Why did the war start?

There was no single cause that makes the outbreak inevitable. Historians debate how much weight to give each factor. Long-term tensions shaped the crisis; the assassination in Sarajevo was its immediate trigger:

• Nationalism: national movements and competition over the status of peoples and states, especially in the Balkans, intensified political tensions.
• Imperial rivalry: competition over territory, influence and resources sharpened distrust among the great powers.
• Militarism and arms competition: large armies, naval expansion and plans for rapid mobilization made leaders more willing to rely on force and less able to pause once a crisis began.
• Alliance politics: states made commitments and strategic calculations that widened the crisis. Alliances did not operate like an automatic switch; governments still made choices during July and August 1914.
• The immediate trigger: on June 28, 1914, Bosnian Serb nationalist Gavrilo Princip assassinated Franz Ferdinand and his wife, Sophie, in Sarajevo. Austria-Hungary blamed Serbia and declared war on July 28. Mobilizations and declarations of war then drew in other powers.

The assassination explains why the crisis began when it did; it does not, by itself, explain why a regional conflict became a prolonged international war.

How did World War I unfold?

• 1914 — a short war becomes a deadlock. Germany invaded Belgium while attacking France. Britain declared war on Germany after the invasion of neutral Belgium. The First Battle of the Marne helped stop the German advance in the west. Opposing armies dug trenches, and the Western Front hardened into a long line.
• 1915–1916 — attrition across several fronts. The war expanded in the Ottoman Empire, the Balkans, Africa and at sea. On the Western Front, battles such as Verdun and the Somme caused enormous losses without a decisive breakthrough. Fighting on the Eastern Front remained more mobile.
• 1917 — the balance begins to shift. The United States declared war on Germany on April 6, citing German submarine warfare and the Zimmermann Telegram. In Russia, revolution toppled the tsar and the Bolsheviks later sought an exit from the war.
• 1918 — offensives, collapse and armistice. After Russia signed the Treaty of Brest-Litovsk in March, Germany shifted troops west and launched the Spring Offensive. Allied counteroffensives pushed the German army back. The Ottoman, Bulgarian and Austro-Hungarian governments sought armistices, and the German armistice took effect at 11 a.m. on November 11.

Why was the war fought around the world?

European empires brought soldiers and resources from colonies and dominions into the conflict. Fighting took place in parts of Africa and the Middle East; Japan entered on the Allied side and seized German possessions in East Asia and the Pacific. Naval blockades and attacks on shipping connected distant fronts to food, fuel and industry at home. The scale varied by region, so “global” should not be mistaken for “the same experience everywhere.”

What were the consequences?

The war killed and wounded millions of soldiers and civilians, displaced people and damaged towns, farms and industry. The German and Austro-Hungarian empires fell; the Russian Empire was overthrown, and the Ottoman Empire broke apart in the years that followed. New states and borders emerged from revolutions, independence movements and postwar treaties.

The November 1918 armistice stopped the fighting between Germany and the Allies; it was not the final peace treaty. The Paris Peace Conference began in 1919. The Treaty of Versailles set terms for Germany, while the League of Nations was created as an attempt to reduce the risk of another war. The settlement left disputes unresolved, and later political and economic crises also shaped the interwar period.

Sources

• National WWI Museum and Memorial — Interactive WWI Timeline.
• National WWI Museum and Memorial — Focused Inquiry: Origins of World War I. On long-term causes and the 1914 trigger.
• National Army Museum — First World War Timeline.
• Imperial War Museums — First World War.
• U.S. Department of State, Office of the Historian — U.S. Entry into World War I.
• U.S. Department of State, Office of the Historian — Paris Peace Conference and Treaty of Versailles.