Steganography played a role in the Revolutionary War and helped George Washington on many occasions. A man by the name of Benjamin Tallmage organized a group of spies in New York, squarely in the middle of British forces. The ring consisted of five people and used the code name Samuel Culper.
Robert Townsend, a reporter from an American newspaper, used his press access to interact with British troops through social functions deemed newsworthy without drawing any undue attention. The Culpers used a series of dead drops, some of which were so elaborate that they occasionally worked against them. At one point, one of the Culpers was caught up in an attack and lost his horse, which carried secret documents. The private letter from Washington mentioned the Cuplers by their code name, but concealed their true identities, which only Tallmage knew.
This incident caused Tallmage to adopt some new security precautions, including invisible inks. James Jay, a doctor who was living in England, invented the ink. James also happened to be the brother of John Jay, who would eventually become the first chief justice of the United States. The invisible ink was used on a blank piece of paper; after the message was written, it was reinserted into a ream of new paper. Washington knew how to find the hidden message by counting from the top down to a specific sheet. He would then apply a second solution to make the ink appear. A concern of Washington's was that carrying around blank sheets of paper would draw suspicion; he ordered that the invisible ink be used on a regular message and the secret message be written between the lines or under the message.
Along with invisible inks, the Culpers used one other method of secret communication. While this method could also be considered cryptography, it is safe to say that it could also fall into the category of steganography and deserves mention.
Washington and the Culpers would use a codebook that had hundreds of common words and would assign a two- or three-digit code to them. Here is a partial sample of the code and translation from George DeWan's paper "Crafty Codes of American Spies":
729 29 15th 1779. Sir. Dqpeu Beyocpu agreeable to 28 met 723 not far from 727 & received a 356 … Every 356 is opened at the entrance of 727 and every 371 is searched, that for the future every 356 must be 691 with the 286 received.
Translated, it reads:
Setauket August 15th 1779. Sir. Jonas Hawkins [an early messenger] agreeable to appointment met Culper Jr. not far from New York and received a letter … Every letter is opened at the entrance of New York and every man is searched, that for the future every letter must be written with the ink received.