<em>The Zimmermann Telegram.</em>
<u>Here is why:</u>
The Zimmermann Telegram was a top secret message from Germany to Mexico which had orders that if Mexico attacked the United States, when Germany won the war, Mexico would get the land back that they previously gave the United States.
Thankfully, Great Britain decoded the telegram and told the United States, who were obviously not happy about this. A speech was given from Woodrow Wilson but the Zimmermann Telegram was not a direct cause of the United States entering World War I, but it was a part of it.
When Woodrow Wilson found out that German submarines were attacking U.S. ships, the line was finally crossed and he declared war. The attacks on United States ships and the Zimmermann Telegram both lead to the United States entering World War I.
