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German U-boats start sinking merchant ships in 1915.
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Shortly after U-boats began sinking merchant ships, Great Britain violates international law by allowing their merchant ships to have guns.
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In response to the Brits' violation of international law, Germany allows its U-boats to sink British merchant ships without warning.
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German U-boats sink British passenger liner Lusitania, angering many Americans with their "barbaric" act.
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Germany pledges to stop sinking passenger ships (like the Lusitania) without any reason.
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The German High Seas Fleet attempts to enter the North Sea and confront the British.
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U-boats torpedo the French passenger ship Sussex.
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In early 1917, Germany was able to break deep into Russia due to the many casualties suffered by the Russians.
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Germany resumed submarine warfare the next day.
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The United States broke off its diplomatic relations with Germany in February 1917.
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Americans intercept the Zimmermann note, which revealed that Germany promised to give Mexicans American land if Mexico declared war on the United States.
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Like Great Britain, the United States began outfitting their merchant ships with guns.
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Woodrow Wilson begins his second term, which he won by the slogan "He kept us out of war."
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The czar was forced to give up the throne to a republican government.
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The City of Memphis, the Illinois, and the Vigilancia were sunk by Germany. Wilson was pretty mad.
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Wilson's Cabinet voted that the United States join the war.
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On April 2, Wilson approached Congress with a plea to join the war, saying that, "The world must be made safe for democracy."
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Congress voted to join the war, and Wilson signed the declaration on April 6, 1917.