What event finally caused the U. S. to enter World War 1?

The Zimmerman telegram was the main reason for the United States entering WWI. Germany's intentions to resume unrestricted submarine warfare was also a reason

There are 5 main reasons why the US joined World War 1 I don't know which reasons you have already listed so I'm just gonna say all 5 the 1st one would be German atrocities in Belgium 2nd would be economic interests 3rd would be sinking of the Lusitania the 4th of the unrestricted submarine warfare and the 5th would be the zinmerman telegram