In 1673 Frenchmen Louis Jolliet and Father Jacques Marquette explored the areas of the Mississippi and Illinois Rivers, and their voyages led to French claims on the region until 1763, when France ceded the land to Great Britain by the Treaty of Paris. During the Revolution, George Rogers Clark and his army won the Illinois Territory for the Commonwealth of Virginia. In 1784 it became part of the Northwest Territory, and on December 3, 1818, Illinois entered the Union as the twenty-first state. U.S. Route 45 enters Illinois at Brookport and proceeds north past the site of Fort Massac, built by the French in 1757 and rebuilt by Americans in 1794. It continues north and east through Harrisburg and through the oil fields of Wayne and Clay Counties. Near Mattoon, the Wisconsin glacier stopped its southern movement. Route 45 bisects the twin cities of Champaign and Urbana, site of the University of Illinois. Kankakee lies along the route and was the site of early French-Canadian migration. North of Kankakee, U.S. 45 passes east of the Joliet Arsenal, which has supplied munitions to the military since 1941. The route skirts Chicago, passing through its western suburbs, and eventually exits Illinois east of Antioch. Along its approximate 230 mile journey through Illinois, Route 45 passes through twenty of the state's 102 counties and eight county seats. Geographically, the route begins on a plane roughly equal to Kitty Hawk, North Carolina, and ends on a plane north of New York City.