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 Illinois Country was a vaguely defined region northwest of the Ohio River which included much of what is now the 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 when France ceded sovereignty of the region east of the Mississippi to the British in the 1763 Treaty of Paris. In the Quebec Act of 1774, the British officially made the Illinois Country part of the Province of Quebec.

In 1778, the population of the Illinois Country consisted of less than 1,000 people of European descent, mostly French-speaking, and about 600 African-American slaves. Thousands of Native Americans lived in villages concentrated along the Mississippi, Illinois, and Wabash Rivers. British official military presence in the region had been nonexistent after the 70 soldiers based at Kaskaskia's Fort Gage had been ordered east during the 1775 Invasion of Quebec. When the soldiers departed, Philippe-François de Rastel, Sieur de Rocheblave, a resident trader and former French officer, was authorized to administer Kaskaskia. Rocheblave, however, lacked the money, resources, and men needed to administer and protect the settlements in the region.

During the Revolutionary War, the Ohio River marked the border between the Illinois Country and Kentucky, which was then a newly settled area claimed by Virginia. The British originally sought to keep Native Americans out of the war, but in 1777, Lieutenant Governor Hamilton received instructions to recruit and arm Indigenous war parties to raid frontier settlements. These war parties were to be accompanied by British Indian Department officers or volunteers from the Detroit militia so as to prevent atrocities. "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."

In 1777, George Rogers Clark was a 25-year-old major in the Kentucky County, Virginia, militia. Clark believed that he could end the raids on Kentucky by capturing the British posts in the Illinois Country and then moving against Detroit. In April 1777, Clark sent two spies to Kaskaskia. They returned two months later and reported that the fort at Kaskaskia was unguarded, that the French-speaking residents were not greatly attached to the British, and that no one expected an attack. Clark immediately wrote a letter to Governor Patrick Henry of Virginia in which he outlined a plan to capture Kaskaskia.

Because the settlers living in Kentucky lacked the authority, manpower, and supplies to launch the expedition themselves, Clark traveled to Williamsburg in October 1777 via the Wilderness Road to meet with Governor Henry. He was joined by a party of about 100 who were leaving Kentucky due to Indigenous raids. Clark presented his plan to Governor Henry on December 10, 1777. To maintain secrecy, Clark's proposal was only shared with a small group of influential Virginians, including Thomas Jefferson, George Mason, and George Wythe. Although Henry initially expressed doubts about whether the campaign was feasible, Clark managed to win the confidence of Henry and the others. The plan was approved by the members of the Virginia General Assembly, who were only given vague details about the expedition. Publicly, Clark was authorized to raise men for the defence of Kentucky. In a secret set of instructions from Governor Henry, Clark was instructed to capture Kaskaskia and then proceed as he saw fit.

Governor Henry commissioned Clark as a lieutenant colonel in the Virginia militia and authorized him to raise seven companies, each to contain fifty men. This unit, later known as the Illinois Regiment, was a part of Virginia's state forces and not a part of the Continental Army. The men were enlisted to serve for three months, once they reached Kentucky. To maintain secrecy, Clark did not tell any of his recruits that the purpose of the expedition was to invade the Illinois Country. Clark was given £1,200 in Continental currency to purchase supplies.