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June 28 - Archduke Franz Ferdinand, heir to Austria-Hungary's throne, and his wife, Sophie, are assassinated by Serbian nationalist Gavrilo Princip while the couple were visiting Sarajevo.
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July 28 - Austria-Hungary declares war on Serbia.
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August 1 - Germany declares war on Russia.
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Germany declares war on France.
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The United Kingdom declares war on Germany, after Germany invades Belgium.
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Germany declares a "war zone" around Great Britain, essentially effecting a submarine blockade where even neutral merchant vessels were to be potential targets.
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The Dardanelles Campaign begins.
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The Second Battle of Ypres begins. It is during this battle that the Germans first use poison gas.
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The Battle of Gallipoli begins.
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The British ocean liner RMS Lusitania is sunk by German U-boat, U-20
