World War I:

Factors leading to U.S. involvement in WWI

Unrestricted Submarine Warfare

Unrestricted Submarine Warfare: The German navy vowed to sink any ship without warning in British waters which resulted in the sinking of five American ships- and the deaths of over 100 American citizens.

The Zimmerman Note

The telegram asked for Mexico to join the Central Powers in the war and in exchange Germany would help Mexico regain territory lost to the U.S. (New Mexico, Texas, Arizona etc.)

Other Factors… U.S. ultimately sides with the British

and Allied Forces due to a common ancestry, language, and similar styles of government.