Pensioner Kenneth Anderson-Jones was rummaging through a car boot sale in his native Stratford-upon-Avon when a dusty, old framed document caught his eye.
The 75-year-old antiques enthusiast bought the item, for just £1, without looking at it too closely. It was only when he got home that he realised its historical significance.
The document is believed to be a lost American treasure - the executive order, dated March 10, 1864, and penned by Abraham Lincoln giving Lieutenant General Ulysses S Grant total command of the United States army, a move that led to victory in the American Civil War.
Mr Anderson-Jones has now contacted Sotheby's and the American Library of Congress to verify that the letter is the 1864 original, which was never recovered.
"I went down to the local market a couple of weeks ago as I always do and there was a young man with a stall who seemed pretty desolate as he wasn't shifting anything," said the pensioner.
"I saw this piece of correspondence in a dirty frame and without really looking at it I offered him £1."
The former gas engineer returned home to his wife, Nancy, to view his find.
"I shouted to my wife, 'Hey look at this'. Behind the dirt was a short document written in ink and signed by Abraham Lincoln.
"I said to my wife 'I wonder how much I would get for an autograph of Abraham Lincoln? Then I read it and thought 'This is really interesting'."