Upon its defeat in the Seven Years War, France ceded her North American possessions east of the Mississippi to Great Britain. Two years later, in 1765, when British troops formally occupied Fort de Chartres, 15 miles upstream from Kaskaskia, the French flag ceased to fly in the Illinois country.

The subsequent British rule was troubled. Many influential French families crossed the Mississippi to avoid contact with the British; and difficulties arose with those who remained because of differences of background, language, and religion. As a belated' solution to the problem, the British ministry in 1774 extended the formal boundaries of Quebec as well as French law and religion into the Illinois country by means of the Quebec Act. This only antagonized traders and speculators who had their own plans for the region, and assured that by the onset of the American Revolution few residents were satisfied with their lot.

With the outbreak of the Revolution in 1776 the "Illinois question" was whether its prairies were to hold the western outpost of a new nation or remain the frontier of a world empire.

Having abandoned Fort de Chartres in 1772, the British removed many of their troops from Kaskaskia in 1776 and left their commander in Illinois begging for money, supplies, and soldiers. Meanwhile, Virginia gave the military responsibility to protect its county of Kentucky to George Rogers Clark. Clark planned to capture Vincennes (Indiana) and nearby Illinois towns from which he hoped to launch an attack on the British headquarters at Detroit. With a force of 175 men he took Kaskaskia without resistance; and on July 4, 1778, the citizens of Vincennes swore allegiance to the colonies before Clark's messengers. On Dec. 9, 1778, Illinois became a county of Virginia.

In retaliation, the lieutenant governor of Detroit, Henry Hamilton (the "hair buyer"), sent British troops to occupy Vincennes. Clark decided upon an immediate mid-winter counterattack and led his men across flooded prairies, through icy water sometimes neck deep. When the Americans reached the Wabash River and employed tactics which combined impressive bluff and military daring, the British troops surrendered Vincennes and its military post, Fort Sackville, on Feb. 24, 1779. Clark and his adventurers thus ended the most famous event of the Revolution in Illinois.

The years 1779 and 1780 were difficult and uncertain in the Illinois country. Clark was sustained only by Oliver Pollack's willingness .to bankrupt himself by accepting the colonel's bills of credit. In the midst of the depreciated currency and of confusion over national loyalty, only minimal civil government was established in the -county" of Illinois when judges were elected at Kaskaskia, Cahokia, and Vincennes in May, 1779. Clark was never sufficiently reinforced to launch his planned attack on Detroit.

By 1780 the French settlers were becoming increasingly resentful of Clark and the Americans. Severe weather complicated government and communication and drove off wild game. Food, clothing, and supplies were scarce. Clark worried about the Spanish controlling New Orleans and the mouth of the Mississippi, the British in Detroit, the fickle French allies, and the indifference of his superior officers in Virginia.

At that point the British launched a coordinated, three-pronged attack on the Illinois country from Detroit, targeting on Kaskaskia, Cahokia, and St. Louis. They hoped to isolate the western outposts from Kentucky. Clark successfully resisted the attack on Cahokia, the Spanish turned back the British and their Indian allies at St. Louis, and the Redcoat advance into Kentucky was ineffective. In pursuit of the attackers of Cahokia, Clark burned the Indian villages at Rock Falls, the westernmost action of the Revolutionary War. Minor incidents in western Michigan concluded the military episodes in the Illinois country.

American control of the western land was tenuous for a decade following war's end in 1783. During this period a number of men who had served with Clark did settle their families at Bellefontaine in present-day Monroe County. While others followed their example, by 10800 there were only 2500 inhabitants in Illinois, approximately half of them French and the other half American.

Virginia had consolidated Clark's conquest by establishing the Illinois country as one of its own counties in December, 1778. Between January, 1782, when county status officially ended, and the creation of the Northwest Territory by the Ordinance of 1787, Illinois was without official government. Governor St. Clair of the Northwest Territory visited Kaskaskia in 1790 to establish the forms of local government to supplement those remnants of Virginia county government which still existed. Government was minimal and unsatisfactory.

In 1800 the Territory of Indiana was established including the Illinois country within its boundaries. Illinois became a territory in its own right nine years later, and a state in 1818.