Which event significantly increased support for American involvement in World War I?

The publication of the Zimmermann Telegram is significant because it revealed a secret communication from Germany to Mexico, proposing a military alliance in the event that the United States entered the war against Germany. This telegram incited outrage among the American public and was a critical factor in shifting public opinion toward supporting U.S. involvement in World War I.

Prior to the telegram's disclosure, many Americans were still ambivalent about entering the conflict, largely favoring neutrality. However, the idea that Germany was seeking to encourage another nation to attack the U.S. directly alarmed many and galvanized support for war efforts. The revelation of the telegram, alongside other factors such as unrestricted submarine warfare by Germany, created a sense of urgency and patriotic duty that led Congress to declare war on Germany in April 1917.