In January 1917, the First World War was not unfolding in the way Germany’s military leaders had imagined. The British naval blockade seemed unbreakable. Something had to change.

Unrestricted submarine warfare appeared to offer a solution. But there was still a problem: U.S. President Woodrow Wilson remained on the sidelines, keeping his country neutral. There was a reason he had just been re-elected in 1916 under the slogan, “He kept us out of war.”

Committed to finding a peaceful solution, Wilson would almost certainly be angered if Germany resumed unrestricted submarine warfare.

Even so, anti-German sentiment in the United States was growing.

Nonetheless, German leaders decided to resume unrestricted submarine warfare on February 1.

They understood the risk.

America might enter the war.

They needed a backup plan.

Tensions between the United States and Mexico offered an opportunity. Mexico could be drawn into an alliance with Imperial Germany. In return, Germany would support Mexico in reclaiming former territories such as Texas and Arizona—lands that had been absorbed by the United States.

Foreign Secretary Arthur Zimmermann was tasked with persuading Mexican President Venustiano Carranza.

The idea was straightforward: Germany would break the British blockade and shift the course of the war in its favor, while Mexico would tie down the United States at its own borders.

But Berlin was not the only capital where frustration was mounting. In London, Admiral William Reginald Hall, Director of Naval Intelligence, was doing everything he could to push the United States toward entering the war.

In January 1917, Hall received word of a decrypted telegram from Room 40, the British codebreaking unit responsible for intercepting and deciphering German communications. When Hall read the message, he immediately recognized its significance.

It was signed by Arthur Zimmermann.

The telegram instructed German officials in Mexico to propose an alliance against the United States.

No sources describe Hall’s immediate reaction to this extraordinary piece of intelligence. But he was an experienced operator. He understood the risks. Revealing the telegram to Washington would expose the fact that Britain was intercepting American communications. Trust could be lost overnight.

That was not an option.

So Hall kept the telegram in reserve.

A few weeks later, circumstances shifted. Germany had resumed unrestricted submarine warfare on February 1. By February 3, President Wilson had severed diplomatic relations with Berlin.

Now was the moment.

Hall knew it—but he still faced a problem. He needed a way to share the information without revealing its source. Opportunities like this rarely come twice.

He revisited the case.

The telegram had been intended for the German ambassador in Mexico, Heinrich von Eckardt. But it had first been sent to Washington, to Ambassador Johann Heinrich von Bernstorff, who was supposed to forward it to Mexico City.

If Bernstorff had done so, there had to be a copy in the Mexico City telegraph office.

Hall reached out to a British field agent known only as “Mr. H.” Acting on Hall’s orders, H. bribed an employee at the telegraph office.There it was: a copy of Zimmermann’s telegram.

Hall immediately passed it to Walter Page, the American ambassador in London. Page, in turn, delivered it to U.S. Secretary of State Robert Lansing.

On March 1, 1917, Lansing released the telegram to the public.

A month later, President Wilson stood before Congress and asked for a declaration of war against the German Empire.

Historians still debate how much the Zimmermann Telegram influenced Wilson’s decision. Decisions of that magnitude are never driven by a single factor. But the message undoubtedly underscored the risks of placing further trust in Germany.

At the same time, the affair reveals something else: how Admiral Hall approached intelligence like a game of chess. He maneuvered carefully, even deceiving the very country he hoped to bring into the war.

There is a reason Hall was appointed Knight Commander in 1918.



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