In July 1778, Clark and his men crossed the Ohio River from Kentucky and took control of Kaskaskia, Vincennes, and several other villages in British territory. The occupation was accomplished without firing a shot because most of the Canadien and Native American inhabitants, who peacefully co-existed with one another, were unwilling to take up arms on behalf of the British Empire. To counter Clark's advance, Henry Hamilton, the British lieutenant governor at Fort Detroit, reoccupied Vincennes with a small force. 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, Virginia.
The importance of the Illinois campaign has been the subject of much debate. Because the British ceded the entire Northwest Territory to the United States in the 1783 Treaty of Paris, some historians have credited Clark with nearly doubling the size of the original Thirteen Colonies by seizing control of the Illinois Country during the war. For this reason, Clark was nicknamed the "Conqueror of the Northwest", and his Illinois campaign—particularly the surprise march to Vincennes—was greatly celebrated and romanticized. Other historians have downplayed the importance of the campaign, arguing that Clark's "conquest" was a temporary occupation that had no impact on the boundary negotiations in Europe.
The Illinois Country was a vaguely defined region which included much of the present U.S. states of Indiana and Illinois. The area had been a part of the Louisiana district of New France until the end of the French and Indian War/Seven Years' War, when France ceded sovereignty of the region to the British in 1763 Treaty of Paris. In the Quebec Act of 1774, the British made the Illinois country a part of its newly expanded Province of Quebec.
In 1778, the population of the Illinois Country consisted of about 1,000 people of European descent, mostly French-speaking, and about 600 African-American slaves. Thousands of American Indians lived in villages concentrated along the Mississippi, Illinois, and Wabash Rivers. The British military presence was sparse: most of the troops had been withdrawn in 1776 to cut back on expenses. Philippe-François de Rastel de Rocheblave, a French-born soldier and official, was hired by the British to be the local commandant, of Fort Gage, in Kaskaskia. Rocheblave reported to Hamilton at Fort Detroit, and frequently complained that he lacked the money, resources, and troops needed to administer and protect the French villages and forts, from internal and external enemies, within the region.
When the American Revolutionary War began, in 1775, the Ohio River marked the border between the Illinois Country and Kentucky, an area recently settled by American colonists. The British had originally sought to keep American Indians out of the war, but in 1777 Lieutenant Governor Hamilton received instructions to recruit and arm Indian war parties to raid the Kentucky settlements, opening a western front in the war with the rebel colonists. "From 1777 on," wrote historian Bernard Sheehan, "the line of western settlements was under almost constant assault by white-led raiding parties that had originated at Detroit."