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Causes of WWI

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Causes of WWI

In June 1914, Franz Ferdinand, the archduke and heir to the Austro-Hungarian Empire, was assassinated in Bosnia. By mid-August, all of the European powers had been drawn into a war.

How did a regional conflict spark a World War? To understand the main causes of World War I in Europe, it's important to look at the sources of increasing tensions in Europe in the years before the war as long term causes of WWI then trace how the assassination of the archduke sparked a general war.

Main Causes of World War I

The main causes of World War I can be summarized in the following list of broad factors:

  • Imperialism and Militarism
  • Nationalism
  • Conflict in the Balkan Region
  • The Alliance System
  • The Assassination of Franz Ferdinand

These factors worked together to provoke a larger conflict when a war broke out between Austria-Hungary and Serbia. It's useful to further consider them in terms of long term causes of WWI and the immediate events that sparked the war before finally considering why the US entered the conflict.


All of the factors above are connected. As you read through this summary, try to consider not only how each was a cause of World War I, but also how each influenced the others.

Long Term Causes of World War I

The main causes of World War I listed above all contributed to the tensions that sparked the war.

Imperialism and Militarism as Cause of World War I

It's important to first consider the role of imperialism and militarism as cause of WWI.

Industrialization Leads to Imperial Conquest and Rivalry

The period before the war had seen the rapid expansion of European empires in Africa and Asia. Imperialism in this period was driven by industrialization. European powers sought the control of raw materials and markets for finished goods.

France and Britain built the largest empires. Meanwhile, Germany wanted a larger empire. There were two crises over Morocco in 1905 and 1911, both of which had enflamed tensions between Britain and France on one hand and Germany on the other.

Militarism and the Arms Race

In the years leading up to the war, all the countries of Europe increased the size of their militaries. A further naval race ensued between Britain and Germany. Each sought to have the largest and most powerful navy.

The Arms Race created a vicious cycle. Each side felt the need to further increase the size of their militaries in response to each other. Larger and more powerful militaries increased tensions and made each side more confident they could win a war.


Nationalism helped to fuel the imperial competition. Countries saw more colonies as a sign of more power. Nationalism also promoted militarism. Nationalists took pride in having a strong military.

Rise of Germany

Germany did not exist as a formal nation state but a loose confederacy of independent states before 1870. These states united behind Prussia during the 1870-71 Franco-Prussian War. A new German Empire was declared after victory in that war. Forged in conflict, militarism became a key part of German nationalism.

Germany quickly industrialized. By 1914, it had the largest army, and its steel production had even surpassed Britain's. Increasingly, the British saw Germany as a threat to be reckoned with. In France, a desire for revenge for the 1871 humiliation further fueled tensions.

Conflict in the Balkans

Nationalism played a different role in fueling tensions in the Balkans region. This area had a mix of ethnic groups that had been long under the control of Austria-Hungary or the Ottoman Empire. Many of them now wanted to be independent and rule themselves.

Tensions were particularly high between Serbia and Austria-Hungary. Serbia had only formed as an independent state in 1878, and it won a series of wars in 1912-13 that allowed it to expand its territory. Austria-Hungary, made up of a variety of ethnic groups and nationalities, including Serbs, saw it as a threat.

Conflict had arisen specifically over the status of Bosnia. Many Serbs lived here, and Serbian nationalists hope to include it as part of a larger Serbia. However, in 1908, Austria-Hungary annexed it. It would be the status of Bosnia that lit the spark of the war.

Causes of WWI Balkan Troubles Cartoon Main Causes of World War I in Europe StudySmarterThe Balkans became known as the "powder keg" of Europe.

The Alliance System

Another one of the main causes of World War I in Europe was the Alliance System. This system had been conceived as a deterrent to war by the German Chancellor Otto von Bismarck. Fearing a possible future war with rival France, he had sought to align Germany with Austria-Hungary. Italy also joined this alliance, creating the Triple Alliance of Germany, Austria-Hungary, and Italy.

Meanwhile, both Britain and France grew increasingly wary of Germany. They announced the Entente Cordiale, or friendly agreement, in 1905. Russia saw itself as a protector of Serbia, which brought it into conflict with Austria-Hungary, while France saw an alliance with Russia as a way to contain Germany. The Triple Entente was the alliance of Britain, France, and Russia.

This Alliance System divided Europe into two competing camps. It meant countries who did not have a direct conflict, such as Germany and Russia, saw each other as rivals. The alliances ensured that a war would not be fought between just two countries but would embroil all of them.

Main Causes of World War I in Europe Map of Alliances StudySmarterA map of the alliances in Europe before World War I. Source: Historicair, CC-BY-SA-3.0, Wikimedia Commons

The Immediate Causes of World War I in Europe

All of the above long term causes of World War I combined with events in 1914 to make a regional conflict between Serbia and Austria-Hungary grow into a broader war.

The Assassination of Franz Ferdinand

Franz Ferdinand was the archduke and heir to the Austro-Hungarian Empire. In June 1914, he visited Sarajevo, the capital of Bosnia.

Serb nationalists plotted and carried out his assassination on June 28, 1924. Austria-Hungary blamed the Serbian government for the assassination. Austria-Hungary declared war on Serbia on July 28, 1914, one month to the day after the assassination.

Alliances Cause a Regional War to Broaden

The invasion of Serbia by Austria-Hungary set in motion the activation of the Alliance System.

Russia Mobilizes

First, Russia mobilized its army in support of Serbia. As their mobilization plans had considered that war with Austria-Hungary would also mean war against Germany, their armies mobilized on Germany's border as well.

In a series of telegrams between Russian Tsar Nicholas II and German Kaiser Wilhelm II, each side expressed their desire to avoid war. However, the Russian mobilization made Wilhelm feel compelled to mobilize his own armies.

The whole weight of the decision lies solely on you[r] shoulders now, who have to bear the responsibility for Peace or War.1" - Wilhelm II to Nicholas II

Germany Activates its War Plans

The Germans were now faced with a decision. Much like Russia, their war mobilization plans were based on an assumption that war with Russia would also mean war with France.

A key factor in German war planning was the desire to avoid a two front war fighting France to the West and Russia to the East at the same time. Therefore, the German war plan, called the Schlieffen Plan, counted on a quick defeat of France by invading through Belgium. After defeating France, the German armies could focus on fighting Russia.

After the French refused to promise neutrality in a war between Germany and Russia, the Germans decided to activate the Schlieffen Plan, declaring war on France and Belgium.

Britain Joins the Fray

Britain responded by declaring war on Germany.

The Alliance System had turned a war between Serbia and Austria-Hungary into a much larger one between Austria-Hungary and Germany, called the Central Powers, on one hand and Russia, France, Britain, and Serbia, called the Allied Powers, on the other.

The Ottoman Empire would later join the war on the Central Powers' side, and Italy and United States would join on the Allied Powers' side.

Causes of WWI Chain of Friendship Cartoon as cause of World War I in Europe StudySmarterCartoon showing the chain of alliances as a main cause of WW1 in Europe. Source: Public Domain, Wikimedia Commons

Causes of US Entry into WWI

There are several causes of US entry into WWI. US President Woodrow Wilson originally declared neutrality. However, the US was eventually drawn into the war.

Relations with Britain and France

The US had a close relationship with Britain and France as allies and trade partners. US banks made large loans to the Allies at the start of the war and the US also sold arms to them.

Furthermore, public opinion in the United States was sympathetic to their cause. Germany was seen as a threat to democracy and reports of German atrocities in Belgium led to calls for intervention.

The Lusitania and Zimmerman Telegrams

More direct tensions with Germany emerged during the war and were also important causes of US entry into WWI.

German U-Boats, or submarines, were highly successful at targeting Allied shipping. The Germans practiced a policy of unrestricted submarine warfare, which meant that they frequently targeted non-military vessels.

One such target was the RMS Lusitania. This was a British merchant ship that was carrying passengers in addition to armaments. On May 7, 1915, the ship was sunk by a German U-Boat. There were 128 American citizens on board, and outrage about the attack was one of the key causes of US entry into WWI two years later.

Another was the Zimmerman Telegrams. In January of 1917, the German Foreign Secretary Arther Zimmerman sent a secret message to the German embassy in Mexico. In it, he proposed an alliance between Germany and Mexico, where Mexico could reclaim the land previously lost to the United States in the event that the US entered the war.

The telegram was intercepted by the British, who turned it over to the US. It sparked national outrage when published in newspapers that March. US entry into WWI followed shortly in April 1917.

The recent course of the Imperial German government... [is] fact nothing less than war against the government and people of the United States...The world must be made safe for democracy.2" -Woodrow Wilson asking Congress to declare war.

Did you know?

Despite its late entry into the war, the US was a crucial player in the negotiations of the Treaty of Versailles that ended the war. Wilson's 14 Points for Peace laid the foundations for the League of Nations and the creation of new nation states in Europe from the old empires before the war.

Causes of WWI - Key takeaways

  • Long term causes of WWI included imperialism, militarism, nationalism, and conflict in the Balkans region.
  • The Alliance System contributed to the causes of World War I in Europe and helped lead to a larger conflict when war broke out between Austria-Hungary and Serbia.
  • Causes of US entry into the war included support for Britain and France and tensions with Germany over events during the war.

1. Wilhelm II. Telegram to Tsar Nicholas II. July 30, 1914.

2. Woodrow Wilson. Speech before Congress asking for declaration of war. April 2, 1917.

Frequently Asked Questions about Causes of WWI

The main causes of WWI were the tensions caused by imperialism and militarism, the alliance system, and the assassination of Austrian Archduke Franz Ferdinand.

The long term causes of WWI included imperial rivalry, conflict in the Balkans region, and the Alliance System.

Militarism was a cause of WWI because each country before the war expanded its military and competed to be the most powerful.

The German signing of an armistice or ceasefire in November of 1917 ended WWI. The Treaty of Versailles formally ending the war occurred in June 1918.

The 4 main causes of WWI were imperialism, militarism, nationalism, and the Alliance System.

Final Causes of WWI Quiz


What countries made up the Triple Alliance?

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Germany, Austria-Hungary, and Italy

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What countries made up the Triple Entente?

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Britain, France, and Russia

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Whose assassination sparked the start of WWI?

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The archduke Franz Ferdinand of Austria-Hungary.

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What two countries went to war to begin World War I?

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Austria-Hungary declared war on Serbia to start World War I.

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Which of the following correctly traces how the war grew into a larger one?

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Germany joined Austria-Hungary to invade Serbia, Russia, Britain, and France then declared war to protect Serbia.

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What are 3 causes of US entry into WWI?

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Support for Britain and France, the sinking of the Lusitania, and the Zimmerman Telegrams.

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What two countries engaged in a naval arms race before the war?

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Britain and Germany

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Why did Britain and France see Germany as a threat?

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Germany quickly industrialized after unifying in 1871 and had the largest army.

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Why were there tensions between Austria-Hungary and Serbia?

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Serbia hoped to include Bosnia, where many Serbs lived, as part of its territory but it was controlled by Austria-Hungary.

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What was the German Schlieffen Plan?

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This war plan called for Germany to defeat France before fighting Russia, which meant it required them to go to war with France when going to war with Russia.

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