In it, Germany’s foreign Secretary Arthur Zimmermann laid out a proposal suggesting that Mexico ally itself with Germany in an attack against the United States, in order to reclaim the territory it had lost in during the Mexican-American war. It also suggested that the ambassador ask Mexico to be a go-between for Japan and Germany.
When Zimmermann wrote the coded telegraph in January 1917 and sent it to the German Ambassador to Mexico, the United States was still neutral in World War I. Woodrow Wilson had been re-elected on a pledge of keeping out of the war, and despite growing anti-German sentiment, few Americans wanted to join the Great War.
The German naval blockade of Britain was not going as well as they’d planned. The Germans would switch to unlimited submarine warfare in just a couple of weeks, a move that would both violate the Sussex agreement and threaten US shipping out of Europe.
In an ultimate twist of irony, the US diplomats in Germany actually held onto and passed along the Zimmermann note, written in code, to other European nations and the State Department in Washington DC. The German government had convinced Woodrow Wilson that passing along its diplomatic messages would be in the interests of peace. When the American ambassador asked about the note, the Germans told him it was part of a peace proposal.
On January 19th, the telegram arrived in Mexico, passed along by Western Union.
What the Germans did not know was that two days earlier, the British intelligence had intercepted the message en-route, and cracked the code. Britain wanted the United States in the war, and on their side. When the British cryptographers in Room 40 read the note, they knew that they had something that would inflame the US popular sentiment against Germany and bring them into the war.
There was one problem, however. If the British government gave the message to the US government, Germany would know that they were able to break German coded messages. Also, they had gotten the message by spying on private American diplomatic messages and they were not sure how the US would react to that breach of trust. So they waited, and the copies of the telegram stayed in a locked desk for the perfect moment.
On February 1st, 1917 Germany lifted their restrictions on submarine warfare and the US government cut their diplomatic ties in protest. In his telegram, Zimmermann had told their ambassador about the plan to unleash unlimited submarine warfare and his prediction that the act would force Britain to surrender within a few months.
If the US joined the war against Germany, then Germany would secretly fund and supply the Mexican invasion to reclaim Texas, Arizona, and New Mexico. Heinrich Van Eckert, the ambassador, was to keep the telegram a secret until the US declared itself. At the time, Zimmermann still hoped that the US would stay neutral.
Noticing that public outrage, the British government finally gave the Zimmermann note to the US government in late February. Even though many Americans were angry about the unlimited submarine warfare, they were not angry enough to demand that they join in the fight. Britain decided the Americans needed an extra push, and that the Zimmermann telegraph would do it.
It did. Outrage over the proposed secret alliance with Mexico won over public opinion and despite his anti-war campaigning, Woodrow Wilson asked Congress on April 2 to declare war against Germany and the Central Powers. With that declaration, the US entered WWI.