Fort de Chartres, the seat of civil and military government in the Illinois country for half a century, was first completed in 1720. Rebuilt of stone in 1753-56 at a cost of 200,000 livres, it became one of the strongest forts in North America and the key to French defenses in what is now the United States. Under the Treaty of Paris in 1763, France ceded most of her North American possessions to Great Britain, but Fort de Chartres, the last post to be surrendered, remained occupied by a French garrison until 1765. Renamed Fort Cavendish, it then served as the seat of British government in the Illinois country until 1772, when it was abandoned and destroyed. The State of Illinois purchased the site in 1913 and made it a state park.