Over 116,000 U.S. citizens died in World War I, making it the third bloodiest war in U.S. history behind World War II and the U.S. Civil War. Though the reasons for the United States’ entry into World War I are many, one of the primary reasons was the Zimmerman telegram, a communique sent from Germany to Mexico, but intercepted and deciphered by British code breakers. The Zimmerman telegram threatened the U.S. territories, thus shifting public sentiment in favor of the Allied Powers of Great Britain, France and Russia.

Offering Help to Mexico

In the Zimmerman telegram, Germany offered support to Mexico so that Mexico could retake territories in Texas and Arizona that were lost in the early 19th century. This support included money to buy arms that would supposedly help in retaking the former Mexican territories. The telegram also sought to strike an alliance between Germany and Mexico, in addition to asking Mexico to help Germany reach out to Japan for additional assistance in the war effort.

The Brazenness of Arthur Zimmerman

Many U.S. officials -- including president Wilson and British foreign minister Walter Page -- were flabbergasted by the brazenness of Zimmerman for sending such a request to the Mexican government. Zimmerman flashed other signs of brazenness when he told U.S. journalists that he did indeed send the coded telegram, and that he assumed U.S. officials would understand he was simply making strategic moves to help secure a German victory. Historians such as Burton Hendrick suggest that Wilson and Page may have pushed for U.S. entry into World War I because they felt personally slighted by Zimmerman’s bold move.

Changing American Sentiments

Prior to learning about the Zimmerman telegram, U.S. citizens had fairly strong anti-Mexican, anti-British and anti-German sentiments. In fact, press coverage of World War I did not portray any “side” as being morally superior to any other “side.” Upon decoding the Zimmerman telegram, U.S. citizens’ attitudes toward the British softened, while their attitude toward Mexicans and Germans hardened. Many U.S. citizens, according to Michael Neiberg, began to see the British as allies, and Germans as enemies. This shift in public sentiment helped ease the United States’ entry into World War I.

Resuming Unrestricted Submarine Warfare

The opening line of the Zimmerman telegram announces Germany’s intention to resume unrestricted submarine warfare against their enemies. This type of naval warfare had already resulted in U.S. casualties, both in terms of the loss of ships and the death of U.S. citizens. U.S. diplomats had threatened Germany to cease unrestricted submarine warfare or face the possible entry of U.S. forces into the war. By declaring their intention to resume such warfare, Germany essentially challenged these diplomats to make good on their promise.