The Zimmermann Note was one of several factors that contributed to the United States' decision to enter World War I, but it was not a direct cause of the war itself. The note, sent by German Foreign Minister Arthur Zimmermann to the German ambassador in Mexico in January 1917, proposed a military alliance between Germany and Mexico in the event that the United States entered the war against Germany. In return, Germany promised to support Mexico in recovering territories lost to the U.S., namely Texas, New Mexico, and Arizona.
When the note was intercepted and made public, it incited anti-German sentiment in the U.S. and helped sway public opinion in favor of entering the war. The revelation of this proposal, along with other factors such as unrestricted submarine warfare by Germany, the sinking of ships like the Lusitania, and economic ties to the Allies, played a significant role in the U.S. decision to declare war on Germany in April 1917.
In summary, while the Zimmermann Note was not a direct cause of World War I, it was a significant factor in the U.S. entering the conflict, which expanded the scope and dynamics of the war.