What significant event influenced the United States to declare war on Germany during World War I?

The significant event that influenced the United States to declare war on Germany during World War I is the submarine warfare in the Atlantic. This form of maritime conflict, particularly Germany's policy of unrestricted submarine warfare, led to the sinking of civilian and merchant ships, which heightened tensions between the United States and Germany. The most notable incident was the sinking of the Lusitania in 1915, where many American lives were lost. Germany’s continuation of such aggressive tactics prompted a strong response from the U.S. government, leading to a shift in public opinion and ultimately a declaration of war against Germany in April 1917. The realization that American lives and interests were at risk played a crucial role in this decision.

While the Zimmermann Telegram also had a significant impact by revealing a German proposal for a military alliance with Mexico against the U.S., the direct threats posed by submarine warfare created an immediate urgency that made the latter a more compelling reason for the U.S. to enter the war.