What two German actions caused the United States to enter world war 1?

Two key German actions that prompted the United States to enter World War I were unrestricted submarine warfare and the Zimmermann Telegram. The resumption of unrestricted submarine warfare in early 1917 threatened American shipping and lives, notably leading to the sinking of the Lusitania in 1915. Additionally, the Zimmermann Telegram, in which Germany proposed a military alliance with Mexico against the U.S., further inflamed public sentiment and pushed the U.S. towards war.