When and Why did America Join in World War 1, the Great War?..
On April 6, 1917, the US entered the Great War (WW1)
On January 9, 1917, German leaders made a decision to inaugurate an all-out submarine war against all commerce, neutral as well as belligerent. The Germans knew that such a campaign would bring the United States into the war, but they were confident that their augmented submarine fleet could starve Britain into submission before the United States could mobilise and participate effectively.
In March 1917, the Germans began to sink American ships indiscriminately, and on April 2, 1917, Wilson asked Congress to recognise that a state of war existed between the United States and the German Empire.
Congress approved the war resolution quickly, and Wilson signed it on April 6, 1917.
The German u-boat sinking of the RMS Lusitania, with 128 US deaths, occurred on Friday, 7 May 1915, almost 2 years before the US entered the war, and was expected, at the time, to bring the US into the war earlier.
Britannica: RMS Lusitania
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