Map of the Phelps And Gorham Purchase
The invasion of the "Genesee Country," or the land of the Six Nations, by Gen. Sullivan and his army in 1779, gave his soldiers and the people of the Colonies an idea of the rich possessions held by the Indians in southern and western New York.
The territory in New York, now composed of the counties of Steuben, Yates, Ontario, part of Wayne, and a large proportion of Monroe, a portion of Genesee and Livingston, and about one half of Allegany, containing about two million six hundred thousand acres of land the Indians claimed, as well as the States of New York and Massachusetts.
On the 21st day of November, 1788, the State of Massachusetts, in consideration of three hundred thousand pounds conveyed to Oliver Phelps and Nathaniel Gorham all its right and title to the above described lands. Phelps and Gorham had previously, in the month of July, 1788, at a council at Buffalo with the Indians, obtained by purchase their interest. [Source]