Henry Hamilton (colonial administrator) in the context of "Fort Detroit"

The Illinois campaign, also known as Clark's Northwestern campaign, was a series of engagements during the American Revolutionary War in which a small force of Virginia militia led by George Rogers Clark seized control of several British outposts in the region northwest of the Ohio River in what is now Illinois and Indiana. The campaign is the best-known action of the western theater of the war and the source of Clark's reputation as an early American military hero.

In July 1778, Clark and his men descended the Ohio River from the Falls of the Ohio, crossed overland to the Mississippi River and took control of Kaskaskia, Cahokia, and several other villages in British territory. Vincennes, on the Wabash River was occupied a few weeks later. The occupation was accomplished without firing a shot because many of the French-speaking inhabitants of the region were sympathetic to the Patriot cause. To counter Clark's advance, Henry Hamilton, the British lieutenant-governor based at Fort Detroit, reoccupied Vincennes with a small force in December 1778. In February 1779, Clark returned to Vincennes in a surprise winter expedition and retook the town, capturing Hamilton in the process. Virginia capitalized on Clark's success by establishing the region as Illinois County.The importance of the Illinois campaign has been the subject of much debate. Because the British ceded the entire area northwest of the Ohio River to the United States in the 1783 Treaty of Paris, some historians have credited Clark actions with nearly doubling the size of the original Thirteen Colonies. For this reason, Clark was acclaimed "Conqueror of the Northwest", and his Illinois campaign—particularly his surprise march on Vincennes—was greatly celebrated and romanticized.