Room 40 revealed Zimmermann Telegram via Mexico copy

In January 1917, British codebreakers at Room 40, the Royal Navy's cryptanalysis unit, decoded a telegram from German Foreign Secretary Arthur Zimmermann proposing a military alliance with Mexico against the United States if America entered the war against Germany.

Publishing the telegram exactly as intercepted risked exposing two things Britain urgently needed to keep secret: that its codebreakers had cracked Germany's diplomatic cipher, which would cause Germany to change codes and cut off a critical ongoing intelligence stream, and that Britain had been intercepting cable traffic passing through or from American commercial lines, a revelation that could seriously damage the diplomatic relationship with the very country Britain needed to draw into the war.

Obtained a duplicate copy sent via commercial cable to Mexico City, and presented that version to the Americans, protecting Britain's codebreaking.

The duplicate was identical, so no content was weakened.

The Mexico copy plausibly leaked locally, not via broken ciphers.

Zimmermann confirmed authorship, so the evidence held up.

The true source stayed hidden, preserving codebreaking and relations.

When acting on privileged info would expose your source, find an alternate, independently plausible path to the same conclusion and act through that.

The Zimmermann Telegram's public release on 1 March 1917 caused the intended outrage in the United States and became a decisive factor pushing America toward declaring war on Germany the following month, while Room 40's codebreaking capability against German ciphers remained undiscovered and Anglo-American relations were not damaged by the true story of how the telegram was obtained.