What event drew the United States into the First World War?  

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The United States was drawn into World War I mainly by the German decision to resume the policy of unrestricted submarine warfare against shipping heading for Great Britain. The British Navy had established a blockade of German ports relatively early in the war, and the Germans resorted to the use of U-boats, or submarines, to retaliate. Germany had previously, after the tragic sinking of the Lusitania as well as other vessels bearing American people and cargo, pledged not to attack American cargo ships without warning. But with the war dragging on and the British blockade taking a real toll on the homefront, the Germans decided to abandon this pledge, and in early 1917, they announced that they were doing so. To make matters worse, the United States (through British spies, who intercepted an infamous telegram sent to the German minister to Mexico) became aware that the German government was making overtures to the Mexican government. Essentially, these overtures requested that Mexico declare war on the United States when the US inevitably declared war on Germany. This so-called "Zimmerman Telegram," along with the renewed threat of submarine war against American vessels, led the United States to declare war against Mexico in April of 1917 (one month after President Woodrow Wilson had been inaugurated for his second term after running on a neutrality platform).