Final answer:
Germany's sinking of the Lusitania and the Zimmerman Telegram were the factors that prompted the US to enter the war.
Step-by-step explanation:
The factor that prompted the United States to enter the war because of Germany's policy of unrestricted submarine warfare was the sinking of the British passenger ship Lusitania.
The sinking of the Lusitania in 1915, which killed 125 Americans, was a significant event that outraged the American public and fueled anti-German sentiment.
Additionally, Germany's attempt to involve Mexico in attacking the US, as revealed in the Zimmerman Telegram, further pushed the United States towards joining World War I.
Learn more about US entry into World War I