Fighting in the West broke out because land-hungry colonists crossed the Appalachian Mountains and settled on Indian territory. During the Revolutionary War, Indians raided white settlements in the wilderness with British encouragement. In 1778, Virginia sent militiamen led by Lieutenant Colonel George Rogers Clark to strike back at the British. Clark captured several settlements in what are now southern Illinois and southern Indiana. The British recaptured the settlement at Vincennes in Indiana. Clark and his men fought their way back to Vincennes across flooded countryside and took its British and Indian defenders by surprise in February 1779.

The community established by the French settlers came under British control in 1763 as a result of the French and Indian War. The British built Fort Sackville at Vincennes in 1777, during the Revolutionary War. In 1778, Virginia troops and local French volunteers commanded by George Rogers Clark occupied Fort Sackville. The British recaptured the fort, but Clark and his forces attacked again and gained control of Vincennes in 1779. Clark's efforts in the West helped the United States win a huge area of land between the Appalachian Mountains and the Mississippi River called the Northwest Territory.