What was germany doing with their u- boats? how did it influence america?

Germany took up a stance of unrestricted submarine warfare, in which Germany sank any ships that was within a certain area regardless of the flag that was waving (whether owned by neutral or warring countries). Because of this, the US was outraged in the fact that their neutral stance was infringed upon, and following with the Zimmermann Telegram, the US officially joined the war on the Allied side.

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