The Zimmermann Telegram (or Zimmermann Note) was a 1917 diplomatic proposal from the German Empire to Mexico to make war against the United States. The proposal was intercepted and decoded by British intelligence. Revelation of the contents outraged American public opinion and helped generate support for the United States declaration of war on Germany in April.

The message came as a coded telegram dispatched by the Foreign Secretary of the German Empire, Arthur Zimmermann, on 16 January 1917 to the German ambassador in Mexico, Heinrich von Eckardt. Zimmermann sent the telegram in anticipation of the resumption of unrestricted submarine warfare by Germany on 1 February, an act which Germany predicted would draw the neutral U.S. into war on the side of the Allies. The telegram instructed Ambassador Eckardt that if the U.S. appeared likely to enter the war, he was to approach the Mexican Government with a proposal for military alliance, with funding from Germany. Mexico was promised Texas, New Mexico, and Arizona. Eckardt was also instructed to urge Mexico to help broker an alliance between Germany and the Japanese Empire. Mexico, much weaker than the U.S., ignored the proposal and (after the U.S. entered the war), officially rejected it.

World War I (WWI), which was predominantly called the World War or the Great War from its occurrence until 1939 (World War II), and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. It involved all the world's great powers, which were assembled in two opposing alliances: the Allies (based on the Triple Entente of the United Kingdom, France and Russia) and the Central Powers (originally centred around the Triple Alliance of Germany, Austria-Hungary and Italy; but, as Austria–Hungary had taken the offensive against the agreement, Italy did not enter into the war). These alliances both reorganised (Italy fought for the Allies), and expanded as more nations entered the war. Ultimately more than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history. More than 9 million combatants were killed, largely because of enormous increases in lethality of weapons, thanks to new technology, without corresponding improvements in protection or mobility. It was the sixth-deadliest conflict in world history, subsequently paving the way for various political changes such as revolutions in the nations involved.