Postal Censorship

Postal censorship is the inspection or examination of mail, most often by governments. It can include opening, reading and total or selective obliteration of letters and their contents, as well as covers, postcards, parcels and other postal packets. Postal censorship takes place primarily but not exclusively during wartime (even though the nation concerned may not be at war, e.g. Ireland during 1939-1945) and periods of unrest, and occasionally at other times, such as periods of civil disorder or of a state of emergency. Both covert and overt postal censorship have occurred.

Historically, postal censorship is an ancient practice; it is usually linked to espionage and intelligence gathering. Both civilian mail and military mail may be subject to censorship, and often different organisations perform censorship of these types of mail. In 20th century wars the objectives of postal censorship encompassed economic warfare, security and intelligence.

The study of postal censorship is a philatelic topic of postal history.

Military mail is not always censored by opening or reading the mail, but this is much more likely during wartime and military campaigns. The military postal service is usually separate from civilian mail and is usually totally controlled by the military. However, both civilian and military mail can be of interest to military intelligence, which has different requirements from civilian intelligence gathering. During wartime, mail from the front is often opened and offending parts blanked or cut out, and civilian mail may be subject to much the same treatment.

...
Wikipedia