Guarding Secrets in the Mail: The World War I Censors

During World War I, words could be as dangerous as weapons. To protect national defense, the British government established a massive intelligence operation known as the Postal and Telegraph Censorship Department. Based primarily in London, this department operated out of large office buildings such as Salisbury House, where thousands of civilian clerks—many of them women and skilled linguists—undertook the meticulous job of examining private correspondence.

Every day, postal examiners sorted through millions of letters, postcards, and telegrams passing between the home front, soldiers in the trenches, and neutral countries abroad. Censors carefully scanned each line of text for sensitive military details, such as ship departure dates, troop movements, ammunition supplies, or references to low soldier morale. If an examiner discovered classified information, they used a thick dark grease pencil to black out the forbidden sentences, or physically excised the offending words with scissors. Highly trained laboratory specialists also treated suspicious pages with chemicals to test for invisible ink used by enemy spies. Once approved, envelopes were resealed with marked paper tape declaring they had been "Opened by Censor" before continuing to their destinations.