Final answer:
German submarines began sinking unarmed merchant and passenger ships without warning in an effort to keep the United States out of the war and secure a quick victory. This pressure eventually led to the United States entering the war.
Step-by-step explanation:
German submarines began sinking unarmed and unresisting merchant and passenger ships without warning in an effort to keep the United States out of the war. The German high command wanted to secure a quick victory by devastating the British economy before the United States could intervene. Their attacks on American ships increased pressure on President Wilson, eventually leading to the United States entering the war. Germany's use of unrestricted submarine warfare played a key role in challenging U.S. neutrality.
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