Which major event caused the involvement of the United States in World War I?

The Zimmerman Telegram is recognized as a significant factor that prompted the United States to enter World War I. This secret diplomatic communication from the German Empire to Mexico proposed a military alliance against the United States. The telegram suggested that if the U.S. entered the war, Mexico would be incentivized to join Germany's side, and in return, Germany would support Mexico in reclaiming territories it had lost to the U.S., such as Texas, New Mexico, and Arizona.

The revelation of the telegram to the American public incited a strong wave of anti-German sentiment and heightened fears of German expansionism. It played a critical role in shifting public opinion in favor of intervention. The combination of this diplomatic crisis, alongside other factors like unrestricted submarine warfare and the sinking of American ships, ultimately led President Woodrow Wilson to call for a declaration of war against Germany in April 1917.

While the sinking of the Lusitania was another crucial event that generated public outrage and pressure for intervention, it was the Zimmerman Telegram that decisively pushed the U.S. government toward a formal declaration of war, making it a pivotal reason for America's entry into the conflict. The assassination of Archduke Ferdinand is linked to the war's ignition in Europe but had no direct connection