A letter from British general John Burgoyne relating to the release of US patriot Ethan Allen is valued at $18,000-24,000 ahead of a sale at Early American History Auctions on August 23.
Allen was the leader of the Green Mountain Boys, a Vermont-based militia that formed to resist encroachment on their territory by the state of New York.
At the start of the revolutionary war, he managed to capture Ford Ticonderoga and went on to participate in the ill-fated invasion of Canada. He was captured in an assault on Montreal in 1775 and shipped to Pendennis Castle in Cornwall, UK.
The lot dates to 1777, a year before Allen was freed in exchange for Scottish colonel Archibald Campbell, and gives a fascinating insight into the negotiations that took place.
It's addressed to American major general Gates and reads in full: "Sir, As your letter of the 28th past did not require an immediate answer I deferred troubling you till the return of Cornet Graffe.
"I conceive that Mr. Ethan Allen is detained or a prisoner of State, but without entering into that consideration, I think it would be inconsistent with the powers or propriety of Sir Henry Clinton's Station or mine, to interfere in a matter which has already been under discussion between Sir William Howe and Genl. Washington.
"If you are inclined, Sir, to make any exchange of Officers or men taken by the Armies under our immediate respective commands, I shall be ready to treat with you.
"I am Sir, Your most obedient Serv't. (Signed) J. Burgoyne."
The sale also includes a sword presented to General Lafayette in 1824 that is expected to make $50,000.
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