On March 1, 1917, the American public learned about a German proposal to ally with Mexico if the United States entered the war. Months earlier, British intelligence had intercepted a secret message from German Foreign Minister Arthur Zimmermann to the Mexican government, inviting an alliance (along with Japan) that would recover the southwestern states Mexico lost to the U.S. during the Mexican War of 1846-47.
The secret to the British interception began years earlier. In 1914, with war imminent, the British had quickly dispatched a ship to cut Germany’s five trans-Atlantic cables and six underwater cables running between Britain and Germany. Soon after the war began, the British successfully tapped into overseas cable lines Germany borrowed from neutral countries to send communications. Britain began capturing large volumes of intelligence communications.
British code breakers worked to decrypt communication codes. In October of 1914, the Russian admiralty gave British Naval Intelligence (known as Room 40) a copy of the German naval codebook removed from a drowned German sailor’s body from the cruiser SMS Magdeburg. Room 40 also received a copy of the German diplomatic code, stolen from a German diplomat’s luggage in the Near East. By 1917, British Intelligence could decipher most German messages.
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What led to the proposal of alliance to Mexico? Zimmermann sent the telegram in anticipation of resumption of unrestricted submarine warfare, an act the German government expected would likely lead to war with the U.S. Zimmermann hoped tensions with Mexico would slow shipments of supplies, munitions, and troops to the Allies if the U.S. was tied down on its southern border.
Some suspected the telegram might be a forgery to manipulate America into the war. However, on March 29, 1917, Zimmermann gave a speech in the Reichstag confirming the text of the telegram and so put an end to all speculation as to its authenticity.
The Zimmermann Telegram galvanized American public opinion against Germany once and for all. The telegram was considered perhaps Britain’s greatest intelligence coup of World War I and, coupled with American outrage over Germany’s resumption of unrestricted submarine warfare, was the tipping point persuading the U.S. to join the war.
Interactive timeline showing 19 events, one at a time. Use the previous and next buttons, or the left and right arrow keys, to move between events. The timeline navigation below lets you jump to any event.
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German Communication Lines Cut
A British cable ship cut Germany’s communication lines, five trans-Atlantic cables, and the six communication cables connecting Great Britain and Germany.
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Codebooks Obtained by British Intelligence
The German SMS Magdeburg, which fired the war’s first shots in the East, runs aground. Russians retrieve three German naval codebooks from the cruiser. One is given to British Intelligence, known as Room 40.
Later in November, a German diplomacy and flag officer codebook was stolen from a diplomat’s luggage in the Near East and also given to Room 40. -
Pancho Villa Raids New Mexico
Francisco "Pancho" Villa, a revolutionary formally backed by the U.S. government, and an army of guerrillas raids and razes the town of Columbus, New Mexico, leaving 19 Americans dead.
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Presidents Wilson and Carranza Convene a Joint High Commission
After raids and increased public anger on both sides of the border, President Wilson and President Carranza convene a Joint High Commission in Connecticut to improve diplomatic relations between the two countries and negotiate the U.S. military presence in Mexico.
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Arthur Zimmermann becomes Germany’s foreign secretary
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Unrestricted Submarine Warfare to Resume
Having suspended attacks in 1916 to avoid U.S. entry into the war, German officials decide to re-engage in unrestricted submarine warfare against all ships.
“I command that unrestricted U-boat warfare shall be instituted with the utmost energy. You are forthwith to make all necessary preparations, but in such a way that this design does not become prematurely apparent to the enemy and to neutrals.”
—Kaiser Wilhelm II, Orders to Admiralty Staff, January 9, 1917
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Proposed Alliance Drafted
A proposal of an alliance with Mexico is drafted by Hans Arthur von Kemnitz, a German foreign office official.
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Telegram Relayed through D.C., then to Mexico
Germany’s foreign minister, Arthur Zimmermann, cables the coded proposal to Germany’s Ambassador in Washington D.C., Count Johann Henrich von Bernstorff. Because of severed transatlantic lines, Germany must rely on neutral nations cable services and the message is routed through Copenhagen.
The telegram reaches the German embassy in Washington D.C. on Jan. 19, and is then relayed to Mexico.
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U.S. Informed of Resumption of Unrestricted Submarine Warfare
The United States is informed of the German High Command order to resume unrestricted submarine warfare.
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United States severs diplomatic relations with Germany
United States severs diplomatic relations with Germany. President Woodrow Wilson tells a joint session of Congress that Germany’s policy of unrestricted U-boat warfare poses an unacceptable threat to “freedom of the seas.”
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U.S. Troops Withdraw from Mexico
The U.S. troops of the Punitive Expedition, after skirmishes with Carrancistas forces and successful diplomacy, withdraw from Mexico.
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Zimmermann Telegram Decoded
The British decode the intercepted message and discover the German proposal for an alliance with Mexico against the United States.
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Germany and Mexico Discuss Telegram
Heinrich von Eckardt, the German envoy to Mexico, discusses the telegram with Mexican foreign minister Cándido Aguilar.
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Translated Telegram Reaches the U.S.; Mexico and Japan discuss the Proposal
U.S. Ambassador Walter H. Page is provided a translation of the telegram on Feb. 23 and informed of Germany’s proposal to Mexico by British Foreign Secretary Arthur Balfour. On Feb. 24, Ambassador Page sends the translation to U.S. Secretary of State Robert Lansing in Washington.
Separately, Foreign Minister Aguilar asks a Japanese legation official in Mexico about Tokyo’s intentions regarding the German proposal.
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U.S. Alerts Mexico and Japan to their Knowledge of Telegram
The State Department instructs the U.S. Ambassador in Mexico to inform the Mexican government that the United States knows about the Zimmermann Telegram. Department counselor Frank Polk discusses the telegram with the Japanese ambassador in Washington.
President Wilson asks Congress for emergency powers to arm merchant ships in response to German unrestricted submarine warfare.
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Japan Will Not Defect From Allies
A Japanese official informs Foreign Minister Aguilar that Japan has no intention of defection from the Allies, having declared war on Germany in August of 1914.
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News Reaches the Public
American newspapers begin publishing the accounts of the “Zimmerman Telegram.” Secretary of State Lansing asks Ambassador Page to verify the decoding of the Zimmermann Telegram because “some members of Congress are attempting to discredit Zimmermann’s message….”
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Zimmermann Verifies Authenticity of Telegram
Zimmermann delivers a speech to the Reichstag confirming the text of the telegram and putting an end to all speculation about its authenticity. By that time more U.S. ships had been torpedoed with heavy loss of life. In late March, Germany sank four more U.S. merchant ships.
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President Wilson Asks for Declaration of War
President Wilson, reelected in November 1916 supporting policies of neutrality, goes before a special session of Congress and asks for a declaration of war.
