Zimmermann Telegram (1917): Germany's Secret Offer to Mexico

The Zimmermann Telegram was a secret German diplomatic message sent on 16 January 1917, proposing a military alliance with Mexico if the United States entered the First World War. Intercepted and decoded by British naval intelligence, it was published in American newspapers on 1 March 1917, and the outrage it caused helped carry the United States into the war that April.

Background: a war that America had kept out of

When war broke out in Europe in 1914, the United States declared neutrality. President Woodrow Wilson won re-election in November 1916 partly on the slogan “He kept us out of war”. American opinion had been angered by German submarine attacks, above all the sinking of the Lusitania in May 1915, but most Americans still did not want to fight.

In Berlin, the military leadership believed that Britain could be starved into submission if U-boats were allowed to sink all ships around the British Isles without warning. The government knew this policy would probably bring the United States into the war. At a conference at Pless on 9 January 1917, Kaiser Wilhelm II approved unrestricted submarine warfare from 1 February.

Germany’s leaders gambled that the war would be won before American troops could arrive in strength. They also looked for ways to keep the United States busy closer to home. Mexico, still unsettled by the Mexican Revolution and resentful of American military incursions such as the Pershing expedition of 1916–1917, seemed a possible partner.

What the telegram said

The message was drafted in the Foreign Office under Arthur Zimmermann, who had become Foreign Secretary in November 1916. It was addressed to the German ambassador in Washington, Count Johann von Bernstorff, for forwarding to the German minister in Mexico City, Heinrich von Eckardt.

The telegram announced that unrestricted submarine warfare would begin on 1 February, while Germany would still try to keep the United States neutral. If that failed, Eckardt was to propose an alliance to Mexico on the basis of making war and peace together, with generous German financial support. Mexico would be allowed to reconquer the territory it had lost in Texas, New Mexico and Arizona in the nineteenth century.

Eckardt was also told to suggest that President Venustiano Carranza invite Japan to join the alliance. The plan was to be raised with Carranza as soon as war with the United States was certain.

How Room 40 intercepted it

Britain had cut Germany’s undersea telegraph cables in the first days of the war. Berlin therefore sent its overseas messages by radio and by borrowed routes. In this case the telegram travelled partly over the American diplomatic cable, which Wilson had allowed Germany to use in the hope of promoting peace talks. That cable passed through British territory.

The British Admiralty’s codebreaking office, known as Room 40, routinely copied this traffic. Under its director, Admiral William Reginald Hall, the cryptanalysts Nigel de Grey and William Montgomery partly decrypted the message on 17 January 1917 and soon grasped its importance.

Hall faced two problems. He did not want the Germans to realise their codes were broken, and he did not want the Americans to learn that Britain was reading their diplomatic cable. The solution was to obtain the copy that Bernstorff had sent on from Washington to Mexico City by commercial telegraph, in an older code. A British agent in Mexico secured this version from the telegraph office, allowing Britain to claim that the text had been obtained in Mexico.

Publication and American reaction

On 23 February 1917 the Foreign Secretary, Arthur Balfour, gave the decoded text to the American ambassador in London, Walter Hines Page. Page cabled it to Washington the next day. Wilson, already angry about the submarine campaign and having broken diplomatic relations with Germany on 3 February, decided to make it public.

The text was released through the Associated Press and appeared in newspapers across the country on 1 March 1917. Some commentators at first suspected a British forgery. That doubt disappeared on 3 March, when Zimmermann publicly admitted that the telegram was genuine, and he defended it again in the Reichstag later that month.

Reaction was strongest in the West and South-West, regions that had been least eager for war but were directly named in the offer. Meanwhile U-boats sank several American merchant ships in March. On 2 April Wilson asked Congress to declare war, and the United States declared war on Germany on 6 April 1917.

Mexico’s answer

Carranza did not reject the proposal at once. He asked a military commission to study it. The commission concluded that Mexico could not hope to win a war against the United States, could not hold the territories even if it took them, and could not count on German arms or money while the Allies controlled the seas.

Carranza formally declined in mid-April 1917. Mexico remained neutral for the rest of the war, although relations with both Germany and the United States stayed complicated. Japan, for its part, was already allied to Britain and showed no interest in changing sides.

Key people

• Arthur Zimmermann (1864–1940): German Foreign Secretary, who approved the plan and later confirmed its authenticity. He left office in August 1917.
• William Reginald Hall (1870–1943): British Director of Naval Intelligence, who managed the interception and timed its release.
• Woodrow Wilson (1856–1924): US president, who published the telegram and took the country into the war.
• Venustiano Carranza (1859–1920): President of Mexico, who considered and then declined the offer.
• Heinrich von Eckardt: German minister in Mexico City, the intended recipient.

Consequences

The telegram did not on its own make war inevitable. The resumption of unrestricted submarine warfare had already led to a break in relations, and American ships were being sunk. But the telegram showed that Germany was willing to encourage an attack on American soil, and it removed much of the remaining opposition to war in Congress and the press.

American entry transformed the war. By November 1918 about two million American soldiers had reached France, and American money and shipping supported the Allies. The episode also became a classic case study in signals intelligence, showing how decoded messages could shape high politics when released with care.

Myths and facts

Myth: the telegram was the sole reason the United States went to war. In fact, most historians see submarine warfare as the main cause, with the telegram acting as a powerful accelerant on public opinion.

Myth: the Americans decoded it themselves. The decryption was entirely British. The Americans received a ready decoded text, and Britain carefully hid how it had been obtained.

Myth: Mexico seriously prepared to invade. Mexico studied the offer but never mobilised against the United States and rejected the alliance.

Timeline

• 1917, 9 January: German leaders approve unrestricted submarine warfare at Pless.
• 1917, 16 January: Zimmermann sends the coded telegram to Washington for forwarding to Mexico.
• 1917, 17 January: Room 40 begins decrypting the intercepted message.
• 1917, 3 February: The United States breaks diplomatic relations with Germany.
• 1917, 23 February: Balfour hands the decoded text to US ambassador Walter Hines Page.
• 1917, 1 March: The telegram is published in American newspapers.
• 1917, 3 March: Zimmermann publicly admits the telegram is genuine.
• 1917, 6 April: The United States declares war on Germany.
• 1917, April: Carranza formally declines the German proposal.

Why it matters

The Zimmermann Telegram is one of the most consequential intercepted messages in history. It helped end American neutrality, shifting the balance of the First World War, and it showed the new power of codebreaking in modern diplomacy and war.