Final answer:
The United States joined WWI due to Germany's unrestricted submarine warfare, the Zimmerman Telegram, and economic motivations.
Step-by-step explanation:
The United States joined WWI primarily due to several key factors. One of the main reasons was the unrestricted submarine warfare conducted by Germany, which threatened American lives and interests. The sinking of the British passenger ship Lusitania in 1915, resulting in the death of 128 Americans, further fueled public outrage and anti-German sentiment in the U.S.
Another significant factor was Germany's proposal of a military alliance with Mexico, known as the Zimmerman Telegram. Intercepted by British intelligence, this message outlined Germany's intention to help Mexico retake territories lost to the U.S. This directly threatened American security and territorial integrity.
Lastly, the United States also had economic motivations to get involved in the war. American banks had loaned large sums of money to the Allies, and if the Central Powers were to win, the U.S. would face significant financial losses.
Learn more about Reasons for the United States joining WWI