During war, keeping troop movements, and movement of naval forces a secret, is very important. There was a slogan used during WWII that helped remind people not to pass information too freely, the slogan was "Loose Lips Sink Ships".
Because it was so important not to let out information that might reach enemy forces, letters were censored. Since letters were routinely censored, most letters contain no real specifics as far as location (except for the country), where the writer is going next, battle content, or movement of other units. All letters were censored after training, whether the writer was a Marine, Army soldier, Navy sailor, or a member of the Coast Guard. As a result, some of the more interesting letters were written after the war ended, or after hostilities ceased in an area.
When a letter had been censored, the person that censored the letter, would usually, put a censor's stamp (usually on the lower left corner), and sign (usually, but not always), or initial near or on the stamp. In the case of some officers, the name of the person who signed the censors stamp, was the same as the person who wrote the letter, meaning I assume, that they were allowed to censor their own letters. Some letters were opened after they had been sealed, and those letters were usually taped back shut, with a tape that said 'opened by censor', or just 'opened by'.