The main effect of the Zimmermann Telegram was how it contributed to the United States joining World War I. The United States was trying very hard to stay out of World War I, President Woodrow Wilson even ran his presidential campaign by using the slogan, "He kept us out of war."
President Woodrow Wilson did not want the United States to join World War I whatsoever, he strived for peace and to keep the country safe. This was challenged when the Lustiana got sank by a German U-boat, which happened to have American passengers on board. This angered a lot of Americans and made them want the United States to join the war, but President Woodrow Wilson still decided to remain neutral. Germany also continued to use unrestricted submarine warfare on multiple occasions, which was another reason the United States joined World War I,
The Zimmermann Telegram was a telegram that was from Germany and going to Mexico, but it got intercepted. This telegram stated that if Mexico were to attack the United States, Germany would give them land when they won the war. This obviously caused outrage among Americans and shortly after, President Woodrow Wilson asked Congress to declare war.