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Arnold Treason Cipher Letter

Benedict Arnold; Peggy Shippen Arnold · 1780

A reproduction of a coded letter Arnold used with his wife's help to secretly negotiate with the British over surrendering West Point.

This coded letter was written by Arnold himself, with his wife's help, to negotiate his betrayal of West Point to the British.

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Printed reproduction of a handwritten page of cipher text on cream paper, with two columns of coded symbols and numbers, signatures at lower right, and the caption 'One of the Treason Letters in Cypher' beneath. A New York Public Library stamp appears at bottom.
Benedict Arnold and Peggy Shippen Arnold, coded treason letter, 1780. New York Public Library.

About this source

What it is

This is a printed reproduction of one of Benedict Arnold's coded letters to the British, written partly in invisible ink. Lines by his wife, Peggy Shippen Arnold, appear between the coded sections written by Arnold himself. It dates from 1780, while Arnold was negotiating to hand over the American fort at West Point.

Why it matters

The letter is direct evidence of the secret plot that made Arnold's name a byword for treason in American history. It also shows that Peggy Shippen Arnold played a role in the scheme, helping disguise the message within her own handwriting.

Look closer

  • Why would a letter about a military secret need two different kinds of writing mixed together?
  • What does it suggest that Arnold needed his wife's handwriting to hide his message?
  • What might the coded numbers and symbols have stood for?

This letter is part of the curated material that grounds the AI version of Benedict Arnold on Humy.ai — when students ask about related events, the AI can quote it and cite it.

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