"when he sold the colony of Louisiana to the United States"
The Louisiana Purchase
Public DomainThe Louisiana Purchase - Credit: US Post Office

In April 1803, Napoleon struck a deal with the United States to sell the colony of Louisiana for approximately $15 million.  The French territory covered 829,000 square miles, stretching from New Orleans into Canada.  The purchase, which included all or part of 14 states, doubled the size of the young nation.

More information about the Louisiana Purchase can be found here.