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Germany invades Belgium and France, leading Britain to declare war on Germany.
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The First Battle of the Marne halts the German advance into France, marking the beginning of trench warfare on the Western Front.
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Serbia refused to accept all of Austria-Hungary's demands on July 25, 1914, when it submitted a response to the ultimatum that agreed to most points but rejected two clauses that infringed upon its sovereignty.
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Archduke Franz Ferdinand and his wife Sophie are assassinated by Gavrilo Princip, a Bosnian Serb nationalist.
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Archduke Franz Ferdinand of Austria is assassinated by a Serbian nationalist.
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Russia officially supported Serbia by ordering a general mobilization of its troops on July 30, 1914, after Austria-Hungary declared war on Serbia on July 28, 1914, in response to the assassination of Archduke Franz Ferdinand.
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Austria-Hungary declares war on Serbia, triggering a network of alliances and beginning World War I.
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Britain and France pledge support to Russia
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Allied forces land on the Gallipoli Peninsula in an attempt to capture the Ottoman Empire's capital, an ultimately unsuccessful campaign.
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A German U-boat sinks the British passenger liner Lusitania, killing many Americans and increasing anti-German sentiment.
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The Battle of the Somme begins, a four-month offensive with huge losses for all sides and little strategic gain.
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Germany begins the massive and brutal Battle of Verdun, a six-month engagement with horrific casualties.
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The United States enters the war after Germany renews its submarine warfare.
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Russia withdraws from the war after signing the Treaty of Brest-Litovsk with Germany.
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The Battle of Amiens and the Meuse-Argonne Offensive result in a decisive Allied victory, pushing back the German forces.
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Germany launches the Ludendorff Offensive, a final attempt to win the war, but the Allies respond with their own offensives.
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An armistice is signed, bringing an end to the fighting on the Western Front.