Final answer:
The Zimmerman Telegram proposed a German-Mexican alliance against the U.S. during World War I, suggesting Mexico invade U.S. territory. It was intercepted by British intelligence, angered the American public, and contributed to the U.S. entering the war against Germany.
Step-by-step explanation:
The Zimmerman Telegram was a secret diplomatic communication issued from the German Foreign Office in January 1917 that proposed a military alliance between Germany and Mexico in the event that the United States entered World War I against Germany. The telegram suggested that if the U.S. were to engage in the war, Mexico should be encouraged to invade the U.S. to reclaim territories lost in the Mexican-American War, such as Texas, New Mexico, and Arizona. Additionally, the telegram mentioned that Germany would seek to include Japan in this alliance.
The offer of territorial gains was intended to keep the U.S. distracted on their own soil, preventing them from sending forces to Europe. This was a strategic move on Germany's part as they were anticipating American entry into World War I on the Allied side. The British intelligence intercepted and decrypted this secret message and the contents were subsequently made public, igniting anger among the American public.
This overt breach of neutrality and the audacious proposal made by Germany were perceived as direct threats to the sovereignty and security of the United States. Moreover, the revelation that this secret negotiation had been attempted by a foreign power with a neighboring country was considered a betrayal and an act of aggression. This led to a surge in anti-German sentiment, thereby influencing American public opinion to support entering the war.
Ultimately, President Woodrow Wilson responded by severing diplomatic relations with Germany in February 1917, and in April, he asked Congress to declare war on Germany, which they did, marking the U.S.'s entry into World War I.