The greatest intelligence coup of World War I was the British interception, decryption, and subsequent exploitation of the Zimmermann Telegram. By January 1917 the war, which had dragged on for two and a half years without either side gaining the upper hand, was reaching a crucial period. Millions had been killed, wounded, or maimed. The internal situation in all the warring countries was declining rapidly. Meanwhile, the United States was trying to stay out of the war. President Woodrow Wilson had proclaimed U.S. neutrality at the war's start and won reelection in 1916 with the slogan "He Kept Us Out of War."And it appeared to the British that, despite a strident prowar group led by Theodore Roosevelt and Senator Henry Cabot Lodge, America would remain on the sidelines unless something drastic happened to unite the American people and propel the country into the war.
The Zimmermann Telegram was that drastic event. On January 16, 1917, Arthur Zimmermann, the German foreign minister, sent a coded message to Ambassador Johann A. von Bernstorff in Washington to be forwarded to German Minister Heinrich J. F. von Eckhardt in Mexico (1) .
That message had been encoded using Code 7500 for transmission to Washington. However, von Eckhardt did not have Code 7500. Upon receipt von Bernstorff decoded it and, after making changes in the date and serial number, recoded it using 13040 (2) .
The British interception of the cable was the result of measures taken immediately after the outbreak of the war to restrict German means of long-distance communications. Before the war, German diplomatic communications to the Western Hemisphere were conducted through various transatlantic cables and by radio. Within a few weeks after the war began all of Germany's means of rapid communication other than radio had been severed as a result of British military and diplomatic action (3) .
Other means of communication were available to the Germans, but there were major drawbacks. The powerful radio station at Nauen, just outside Berlin, was a target of four British listening posts. An even less desirable method involved the receiving terminals at Sayville, Long Island, and Tuckerton, New Jersey. In accordance with U.S. neutrality, all messages transmitted or received at the stations had to be submitted to the U.S. government for censorship, along with the code or codes and the plaintext copies of each message (4) .
Another means of communication was provided by the Swedish government. Messages were transmitted from Berlin to Stockholm by radio and then, by cable, to Buenos Aires and on to Washington. The messages appeared as Swedish message traffic, although they were actually in German code to which a superficial encipherment had been added (5) .
The final method available to the Germans for radio communication was through the U.S. government. During some critical periods, encrypted German communications were transmitted from Berlin to Washington and Washington to Berlin by the State Department. The communications, masked as U.S. traffic, were sent without the United States being furnished either plaintexts or the codes. The Germans persuaded the State Department to consent to this method on the grounds that it was the only means to ensure the security of German communications between Berlin and Washington. The German messages, however, were not recoded into the State Department code but simply given the preamble, address, and signature of a U.S. message; since the U.S. code usually consisted of letter groups while the German code consisted of numbers, the British could easily distinguish the German messages from actual U.S. message traffic (6) .
The importance of this message led Zimmermann to take special measures to make sure it reached its destination. Originally it was to be sent via Sweden and by a submarine, the Deutschland. The Deutschland was, however, delayed and Zimmermann chose as the second route for his message the State Department, sending what appeared to be a revised message concerning his reaction to President Wilson's recent peace proposals (7) .
On the morning of January 17, two days before the message would be relayed from Washington to Mexico, William Montgomery and Nigel de Grey, two cryptanalysts working in Room 40's diplomatic section, handed a partially decoded copy of the telegram using 7500 to Blinker Hall. At the time the British had been working on both codes. While they had solved most of 13040, they had been much less successful with 7500, which was more complex in nature and had been in existence for a much shorter period of time. In addition, traffic sent in 7500 was limited because the code had not been widely distributed (8) .
Room 40's inability to master 7500 was evident by the many unsolved sections in the partial text they produced from the intercepted telegram:

Most secret for your Excellency's personal information and to be handed on to the Imperial Minister in (Mexico?) with Tel. No. 1 *** by a safe route.
We propose to begin on 1st February unrestricted submarine warfare. In doing so, however, we shall endeavor to keep America neutral ***? If we should not (succeed in doing so), we propose to (Mexico?) an alliance upon the following basis:
(joint) conduct of war
(joint) conduct of peace
Your Excellency, should for the present inform the President secretly (that we expect) war with the U.S.A. (possibly) ( *** Japan) and at the same time to negotiate between us and Japan *** (indecipherable sentence meaning please tell the President) that *** our submarines will compel England to peace in a few months. Acknowledge receipt.
Zimmermann (9)

Montgomery and de Grey, zealously attempting to uncover the total message, were able to produce only the partial text. The significance of the intercept was immediately understood by Hall. The message, even in its garbled form, might push the United States into war. However, providing the United States with the message would force Hall to reveal how the message was obtained and also constitute an admission that the British were reading the messages of neutral nations, which included the United States. The latter implication might cause resentment and jeopardize Britain's efforts to enlist the United States as an ally. In addition, providing the message in its garbled form could cause doubts about its authenticity and even result in charges that Britain had created the entire telegram (10) .
On the other hand, if Hall could make it appear that the telegram was acquired in Mexico, the Germans might conclude that only the plaintext version delivered to von Eckardt had been uncovered. In that case the Germans would presumably continue to use their current codes and Room 40 could continue reading them. Further, Hall may have known or assumed that Eckardt did not have Code 7500 and that von Bernstorff might be forced to relay Zimmermann's message in a more breakable code, resulting in a complete version of the telegram (11) .
In early February a British official in Mexico, through an agent in the telegraph office in Mexico City, acquired the telegram. And the telegram had indeed been encoded with a code that Room 40 had already solved, Code 13040 (12) .
The full telegram read:

We intend to begin unrestricted submarine warfare on the first of February. We shall endeavor in spite of this to keep the United States neutral. In the event of this not succeeding, we make Mexico a proposal of alliance on the following basis: Make war together, make peace together, generous financial support, and an understanding on our part that Mexico is to reconquer the lost territory in Texas, New Mexico, and Arizona. The settlement in detail is left to you.
You will inform the President [of Mexico] of the above most secretly as soon as the outbreak of war with the United States is certain and add the suggestion that he should, on his own initiative, invite Japan to immediate adherence and at the same time mediate between Japan and ourselves.
Please call the President's attention to the fact that the unrestricted employment of our submarines now offers the prospect of compelling England to make peace within a few months. Acknowledge receipt.
Zimmermann (13)
Although the British had the full version in hand, with its suggestion of Mexican recovery of Texas, New Mexico, and Arizona, they were still hesitant about passing it to the United States. But even after the early February break in relations with Germany over unrestricted submarine warfare, there were no indications that the United States would enter the war any time soon (14) .
Finally, Britain could delay no longer. On February 22 Hall showed the decoded Bernstorff-to-von Eckhardt telegram to Edward Bell, the American embassy official who handled liaison with the British intelligence services. Two days later Ambassador Walter Hines Page telegraphed the contents to Wilson, with the fanciful explanation of its interception and decipherment that Hall had given Page. As might be expected, the British put no restriction on the publication of the telegram (15) .
The New York Times headline on March 1 read: "Germany Seeks An Alliance Against US; Asks Japan and Mexico to Join Her; Full Text of Her Proposal Made Public." The full text of Zimmermann's message was reprinted immediately below the headline. The Washington Post headline proclaimed: "German Plot to Conquer United States with Aid of Japan and Mexico Revealed: Details of Machinations, Begun in Berlin January 19, Furthered by Von Bernstorff, in President's Hands (16) .
The impact was predictable and overwhelming. But some senators objected that the authenticity of the telegram had not been fully verified.
As a result Secretary of State Robert Lansing telegraphed Page: "This government has not the slightest doubt as to its authenticity, but would be of the greatest service if the British Government would permit you . . . to personally decode the original message which we secured from the telegraph office in Washington . . . Assure [Foreign Minister Arthur Balfour] that the Department hesitated to make this request but feels that this course will materially strengthen its position and make it possible for the Department to state that it has secured the Zimmermann note from its own people." (17)
On March 2 Page's reply informed Lansing that Bell had personally decoded the Zimmermann telegram and would testify to its legitimacy. As it turned out Bell's testimony was not necessary. On March 3, for reasons that remain unknown, Zimmermann eliminated all doubts when he acknowledged the authenticity of the telegram. A little over a month later, on April 6, 1917, the United States entered the war. (18)
The United States would probably have joined Britain anyway, as a consequence of Germany's unrestricted submarine warfare. However, not everyone was affected by such warfare. Further, by the time the United States would have been ready to enter the war the Central Powers might have been victorious. But the telegram posed the clear threat of the loss of U.S. territory in the Southwest and West, making Germany a concrete menace to the people living there. It also led many to believe that a German victory would mean German annexation of some parts of the country. It clearly indicated German hostility and helped convert "Europe's war" into a "Great War for Democracy. " (19)
Secretary of State Lansing concluded that the telegram was more important than Germany's resumption of unrestricted submarine warfare in unifying American public opinion and putting "the people solidly behind the Government" and making war "inevitable, if not popular, because the German Government's sinister intent toward the United States should no longer be doubted." (20)