Answer:
When World War I broke out across Europe in 1914, President Woodrow Wilson proclaimed the United States would remain neutral, and many Americans supported this policy of nonintervention. However, public opinion about neutrality started to change after a German U-boat sank the British ocean liner Lusitania in 1915; almost 2,000 people perished, including 128 Americans. Along with news of the Zimmermann telegram threatening an alliance between Germany and Mexico against America, Wilson asked Congress for a declaration of war against Germany. The United States officially entered the conflict on April 6, 1917.
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