Final answer:
The U.S. entered World War I primarily due to Germany's unrestricted submarine warfare causing American casualties and the revealing of the Zimmerman Telegram, which threatened U.S. territorial integrity, alongside strong economic ties with the Allies.
Step-by-step explanation:
The main reason the U.S. decided to enter the war on the Allied side was a combination of Germany's unrestricted submarine warfare and the Zimmerman Telegram. Germany announced unrestricted submarine warfare, which resulted in the loss of American lives and ships, further souring the relations between the two nations. The Zimmerman Telegram, which proposed a military alliance between Germany and Mexico in the event of the U.S. entering WWI against Germany, caused public outrage and was seen as a direct affront to the United States' sovereignty and national security. Additionally, there were strong economic ties to the Allies, and with the Allied powers facing exhaustion, the U.S. entry provided much-needed support.