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July 28: The assassination of Archduke Franz Ferdinand of Austria and his wife Sophie von Chotek in Sarajevo, Bosnia, sparks the outbreak of World War I.
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May 7: The Lusitania, a British passenger liner, is torpedoed by a German submarine off the coast of Ireland, killing over 1,100 people, including 128 Americans. This event heightens anti-German sentiment in the United States.
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March 1: President Woodrow Wilson issues a proclamation of neutrality, declaring that the United States will not take part in the war.
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May 31: The Zimmermann Telegram, a secret diplomatic communication from Germany to Mexico proposing a military alliance against the United States, is intercepted and decoded by the British. The telegram is later leaked to the American press, further eroding American neutrality.
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February 3: Germany resumes unrestricted submarine warfare in the Atlantic, declaring that all ships, regardless of nationality, entering the designated war zone will be subject to attack.
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February 24: Three American merchant ships are torpedoed by German submarines in a single week, killing scores of American citizens.
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April 2: President Wilson addresses a joint session of Congress, asking for a declaration of war against Germany. He argues that Germany's actions have made neutrality impossible and that the United States must join its allies to make the world "safe for democracy."
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April 6: Congress votes to declare war on Germany, with the Senate approving the measure by an overwhelming margin of 82-6.
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April 20: President Wilson signs the war declaration into law, officially bringing the United States into World War I.
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April: The American Expeditionary Force (AEF), led by General John J. Pershing, begins its offensive operations in France.
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January 8: The United States Army forms its first complete division to arrive in Europe, the 1st Division.
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September: The Allies launch a major offensive against Germany, with the American forces playing a significant role.
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November 11: The Armistice is signed between the Allies and Germany, officially ending World War I. The United States, though having joined the war late, had made significant contributions to the Allied victory.
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January: President Wilson attends the Paris Peace Conference, where he advocates for his vision of a League of Nations to prevent future wars.
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June: The Treaty of Versailles is signed, officially ending the state of war between Germany and the Allied Powers. The United States Senate, however, ultimately rejects the Treaty and the League of Nations, marking a significant turning point in American foreign policy.