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Archduke Franz Ferdinand is assassinated in Sarajevo. His death is the event that sparks World War I.
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Russia mobilizes its army to intervene against Austria-Hungary in favor of its ally, Serbia. The move leads to the mobilization of the rest of the European Great Powers, and inevitably to the outbreak of hostilities.
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Germany invades Belgium, beginning World War I.
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The Germans fire shells filled with chlorine gas at Allied lines. This is the first time that large amounts of gas are used in battle, and the result is the near-collapse of the French lines.
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A German submarine sinks the passenger liner Lusitania. The ship carries 1,198 people, 128 of them Americans
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Reacting to international outrage at the sinking of the Lusitania and other neutral passenger lines, Kaiser Wilhelm suspends unrestricted submarine warfare.
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The British employ the first tanks ever used in battle, at Delville Wood.
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Germany resumes unrestricted submarine warfare in European waterways.
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British intelligence gives Wilson the so-called Zimmermann Telegram, a message from German foreign secretary Arthur Zimmermann proposing that Mexico side with Germany in case of war between Germany and the United States.
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President Wilson outlines his case for war to Congress.
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Congress authorizes a declaration of war against Germany. The United States enters World War I on the side of France and Britain.
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Congress passes the Selective Service Act authorizing the draft. Although criticized for destroying democracy at home while fighting for it abroad, President Wilson claims he sees no other option and signs the bill into law.
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The Germans sign a peace treaty with the new Bolshevik government of Russia. The treaty give Germany land that had been the Ukraine and Poland, and peace on the Eastern Front allows Germany to shift soldiers to the Western Front, causing problems for the French, British, and Americans.
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The Battle of Cantigny is the first major American offensive of the war.
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The Battle of St. Mihiel begins when 300,000 American troops under the direct command of General Pershing fling themselves into the German lines.
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