George Rogers Clark in the context of "Indiana Territory"

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.

The siege of Fort Sackville, also known as the siege of Fort Vincennes or the Battle of Vincennes, was an American Revolutionary War frontier battle fought in present-day Vincennes, Indiana. In February 1779, an American militia led by Lieutenant Colonel George Rogers Clark defeated the British garrison of Fort Sackville commanded by Lieutenant Governor Henry Hamilton. Clark surprised the fort after a gruelling march from Kaskaskia, and following the execution of four Odawa captives, forced Hamilton's surrender.

Illinois County, Virginia, was a political and geographic region, part of the British Province of Quebec, claimed during the American Revolutionary War on July 4, 1778, by George Rogers Clark of the Virginia Militia as a result of the Illinois Campaign. Though part or all of the area was also claimed by Connecticut and Massachusetts, it was formally organized by the Commonwealth of Virginia later that year. The County was accorded official governmental existence, including legally defined boundaries and a formal governmental structure under the laws of the Commonwealth. The county seat was the old Illinois Country French village of Kaskaskia. John Todd was appointed by Governor Patrick Henry to head the county's government. The county was abolished in January 1782, and Virginia ceded the land to the new United States Confederation government in 1784. The area later became the Northwest Territory by an Act of Congress in 1787.

Geographically, the county was bordered to the southeast by the Ohio River, in the west by the Mississippi River, and in the north by the Great Lakes at the time of its existence. It included all of what were known as eastern Illinois Country under French sovereignty and Ohio Country. Politically, its effective reach extended only to the old French settlements of Vincennes, Cahokia, and Kaskaskia.

Fort Kaskaskia State Historic Site is a 200-acre (0.8 km) park near Chester, Illinois, on a blufftop overlooking the Mississippi River. It commemorates the vanished frontier town of Old Kaskaskia and the support it gave to George Rogers Clark in the American Revolution.