Benedict Arnold's Betrayal - Letter of May 10,

Benedict Arnold’s intended betrayal was carefully planned for more than a year before John André was captured and charged as a spy. 

This first page (of a four-page letter), which André sent to Arnold’s go-between (Joseph Stansbury), makes clear that it was Arnold who approached the British, not the other way around. 

Referring to General Arnold as "Monk," André writes:

On our part we meet [deleted ArnGen] Monk's ouvertures with full reliance on his honourable Intentions and disclose to him with the strongest assurances of our Sincerity, that no thought is entertained of abandoning the point we have in view.

Click on the image for a slightly better view.

Credits

Image, Clinton Collection at the Clements Library, University of Michigan.