Question

What was one of the events that led to U.S. involvement in WWI?

Answer 1

There were a few reasons but the main reason was the act of Germany carrying out unrestricted submarine warfare.

This meant that German submarines would sneak attack other submarines or ships, including US property, without warning.
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Answer 2

There are a few reasons why the U.S. (Despite how hard they tried) could not remain neutral. One of the main reasons was because the Germans were sinking ships, many of which belonged to the U.S. Another big reason was because the Germans tried to get Mexico to declare war on the U.S. which ultimately forced the U.S. to get involved. 

Hope this helped. Have a great day!