The Zimmerman Telegram: The secret plot to attack America in World War I

On 16 January 1917, Arthur Zimmermann was Germany’s Foreign Secretary and dispatched a coded message to Heinrich von Eckardt, the German ambassador in Mexico, instructing him to offer the Mexican government a risky deal.

If the United States entered the war against Germany, Mexico was to join the Central Powers and declare war on the United States.

In return, Germany promised financial assistance and political support to help Mexico recover Texas and Arizona, as well as New Mexico, territories that it had either lost or seen annexed during the nineteenth century.

Texas, for example, had declared independence from Mexico in 1836 and was annexed by the United States in 1845, prior to the Mexican-American War.

The message included the specific wording: “We make Mexico a proposal of alliance on the following basis: make war together, make peace together.” 

At its core, the telegram essentially outlined a plan to divert American attention away from the European theatre so that a second front would open in North America.

German leaders assumed that the prospect of a war with Mexico would likely force the United States to keep troops at home and significantly delay any direct involvement in Europe.

According to Zimmermann’s instructions, the proposal would only be activated upon confirmation of American entry into the war, but the message revealed a clear willingness to involve the Western Hemisphere in Germany’s larger strategy for victory. 

To Germany, the United States represented an increasingly serious threat.

Although it had not yet declared war, American industries had supplied Britain and France with arms and loans, together with other matériel that supported their war effort.

German officials feared that such support would soon shift from indirect aid to direct military engagement.

So, the telegram aimed to counter this possibility by sowing chaos in America’s backyard.

At the start of 1917, the German high command faced an increasingly serious and dangerous situation.

The war in Europe had reached a deadlock, and while Russia remained bogged down by internal strife, the Allies continued to receive supplies from overseas.

To force a breakthrough, German naval leaders persuaded Kaiser Wilhelm II to authorise the resumption of unrestricted submarine warfare.

On 31 January, Germany announced it would sink all ships, including neutral and American vessels, in the waters that surrounded Britain and its allies. 

By extension, German leaders accepted that this decision would provoke the United States.

However, they calculated that American forces would need many months to raise and equip an effective army before deploying it overseas.

During that time, the much stronger U-boat campaign might severely cripple Britain’s supply lines and possibly force a peace settlement.

On 3 February, the German submarine sank the American freighter SS Housatonic off the coast of Scilly, increasing tensions further.

Some sources identify U-53 as the submarine responsible, though confirmation remains uncertain. 

At the same time, German foreign policy increasingly turned to sabotage and political disruption.

The idea of encouraging Mexico to attack the United States originated from the hope that a southern threat would slow down or prevent American military involvement overseas.

German officials misread the situation in Mexico because they believed the country’s revolutionary government might welcome the opportunity to reclaim lost land and settle old grievances.

This is because Carranza’s regime had been recently established and appeared to offer Germany a potential ally in the Western Hemisphere. 

Crucially, Zimmermann and his staff underestimated the political instability and weak economy of Mexico.

Internal divisions remained unresolved, and the Mexican military was still poorly equipped for a foreign campaign that would involve fighting the United States.

Despite this, the telegram went ahead as a strategic move designed to stretch American attention thin and gain any advantage possible before time ran out.

Soon after the message had left Berlin, it passed through a series of telegraph relays, including one routed through American diplomatic cables that the United States had permitted Germany to use for encrypted communications.

Because those lines ran through Britain, British intelligence agents monitored transatlantic cables and took the chance.

Inside Room 40, the secret Admiralty codebreaking unit led by Admiral Sir William Hall employed analysts who intercepted and began decoding the telegram almost immediately. 

Although the message used a complicated code, British codebreakers had already obtained some access to Germany’s diplomatic codebooks and made swift progress.

Within days, codebreakers such as Nigel de Grey and William Montgomery pieced together the key phrases.

By 19 January, the general purpose of the telegram had become known, and by 5 February, its full contents had been revealed.

British intelligence now had clear proof that Germany was urging Mexico to wage war against the United States. 

However, British officials hesitated to make public what they had found. If Germany realised its diplomatic codes had been compromised, it might change them, which would shut down a very important source of wartime intelligence.

Additionally, they needed to present the information to the Americans in a way that would not make the British look as if they were trying to trick the Americans. 

To overcome this, British agents obtained a second version of the telegram, which had been forwarded from the German embassy in Washington to Mexico City with Code 0075, a less secure cipher used for embassy traffic.

This alternative version allowed them to claim that the telegram had been acquired by more conventional means, and this protected their secret codebreaking efforts and also provided the Americans with what appeared to be a verifiable document.

On 23 February, the British handed the decrypted message to Walter Hines Page, the American ambassador in London.

The next day, President Woodrow Wilson received the document and read the details for himself.

Until that moment, he had tried to preserve American neutrality, even as German submarines had already sunk several American ships.

The telegram presented a new and very hard choice: if the American public learned that Germany had plotted to incite an attack on U.S. territory, the pressure to go to war would be immense. 

Wilson and his advisers understood that releasing the message would shift public opinion.

Still, they had to consider the risks. Some might question whether the telegram was genuine, and others might accuse the administration of inventing an excuse for war.

Even so, the President judged that the American people had the right to know what Germany had done.

On 3 March, Zimmermann confirmed the telegram's authenticity during a press conference. 

On 1 March, the telegram appeared in newspapers across the country, and headlines carried the shocking news that described the contents of the message.

In cities and towns from coast to coast, many Americans read about Germany’s secret offer to Mexico and responded with fury.

The New York Times ran the headline “Germany Seeks Alliance Against U.S.” while The Chicago Tribune condemned Germany’s betrayal in stark language.

For many, the idea that a European power would conspire to divide the United States and support a foreign invasion crossed a line that could not be ignored.

Across political and regional lines, anger spread quickly. Readers who had previously opposed the war now demanded action, while isolationists found themselves on the defensive.

Even German-American communities expressed anger and distanced themselves from Berlin’s scheme. 

For Wilson, the timing proved critical, as the telegram arrived as unrestricted submarine warfare resumed and American lives continued to be lost at sea.

Together, the two events shifted the national mood. Wilson framed Germany’s actions as a betrayal of civilised diplomacy and painted the United States as a reluctant power forced into war to defend its honour. 

Meanwhile, newspapers ran editorials that condemned Germany’s dishonesty. The image of a foreign power that tried to use America’s neighbour as a weapon turned general fears into a clear threat.

Anti-German sentiment, which had already been growing since the Lusitania disaster, now reached noticeably new heights.

Military enlistment increased, and calls for mobilisation echoed in the press. By April, over seventy Americans had been killed in U-boat attacks since February. 

By mid-March, most of the American political establishment had come to support intervention.

In fact, the Zimmermann Telegram became the final piece in the case Wilson needed to present to Congress.

On 2 April 1917, Wilson stood before a joint session of Congress and requested a declaration of war.

He argued that Germany had turned the seas into a war zone and now tried to unleash violence upon American soil.

He referenced the telegram directly, presenting it as proof of Germany’s aggressive intent.

Four days later, Congress voted to declare war by 373–50 in the House of Representatives and 82–6 in the Senate, which is now considered to be official point that the United States’ entered into the global conflict. 

From that moment, America’s military machine began to mobilise on a large scale, as Congress passed the Selective Service Act on 18 May, which allowed the government to conscript millions of men.

War industries expanded very rapidly, while American Expeditionary Forces prepared for deployment to France.

Under the leadership of General John J. Pershing, troops began to arrive in Europe in mid-1917, and full-scale participation began the following year.

In Mexico City, the reaction to the telegram was more practical than emotional.

President Venustiano Carranza reviewed the proposal and promptly referred it to his military staff.

General Álvaro Obregón led the review and concluded that war with the United States would be terrible.

Mexico had remained weakened by civil conflict and had neither the resources nor the stability to engage in such a campaign. 

Additionally, Carranza’s advisers doubted that Germany could deliver the promised support, since the British blockade had cut off most of Germany’s overseas reach, and any aid would likely arrive too late or not at all.

Carranza also recognised that the territories Germany offered, including Texas and Arizona as well as New Mexico, contained large, well-armed populations unlikely to submit to Mexican rule. 

Mexico’s wariness stemmed in part from earlier German intrigues. During the U.S. occupation of Veracruz in 1914, Germany had attempted to smuggle weapons to Mexican revolutionaries.

That experience reinforced doubts about Berlin’s reliability. 

Rather than ignore the offer entirely, Carranza responded formally to Germany by noting that the plan was not viable under Mexico's current circumstances.

Then, when the telegram became public, the Mexican government downplayed its relevance and avoided direct comment.

In this way, Mexico avoided further involvement and denied Germany the distraction it had hoped to provoke. 

Ultimately, the Zimmermann Telegram largely failed to achieve its intended goal.

Rather than isolating the United States, it helped push Washington into action. As a piece of wartime diplomacy, it became a case study in a very serious mistake.