Final answer:
The United States joins the Allies by declaring war on Germany after Germany resumes unrestricted submarine warfare and issues the Zimmerman note.
Step-by-step explanation:
The United States joins the Allies by declaring war on Germany after Germany resumes unrestricted submarine warfare and issues the Zimmerman note. The release of the Zimmerman Telegram to the US public on March 1 added to the sense of rage and produced the sentiment that the United States was honor bound to respond with military force. American public sentiment finally shifted and the US declared war on Germany in April of 1917.