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.

To counter Clark's advance, Henry Hamilton, the British lieutenant-governor based at Fort Detroit, reoccupied Vincennes with a small force in December 1778.

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.

[2] 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.

[4] 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.

[5] "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.

[7] 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.

[8] 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.

[11] Governor Henry commissioned Clark as a lieutenant colonel in the Virginia militia and authorized him to raise seven companies, each to contain fifty men.

[20] After repeated delays to allow time for more men to join, Clark left Redstone by boat on May 12, 1778, with about 150 recruits, organized in three companies under captains Bowman, Helm, and Harrod.

[21] Clark and his men picked up additional supplies at Fort Pitt and at Henry that were provided by Brigadier General Edward Hand, the Continental Army Western Department commander.

[23] Clark stopped at the mouth of the Kentucky River and despatched a message upriver to Major Smith, telling him that it was time to rendezvous.

The news was greeted with enthusiasm by many, but some of the Holston men deserted that night; seven or eight were caught and brought back, but others eluded capture and returned to their homes.

[25] While Clark and his officers drilled the troops in preparation for the Kaskaskia expedition, the families who had traveled with the regiment down the Ohio River settled on the island and planted corn.

[27] While on the island, Clark received an important message from Pittsburgh: France had signed a Treaty of Alliance with the United States.

[29] Clark's force numbered about 175 men in total, organized in four companies under Captains Bowman, Helm, Harrod, and Montgomery.

Because Clark hoped to take Kaskaskia by surprise, he decided to march his men across what is now the southern tip of Illinois and approach the village by land, a journey of about 120 miles (190 km).

Father Pierre Gibault, the village priest, was won over after Clark assured him that the Catholic Church would be protected under the laws of Virginia.

On the afternoon of July 5, Captain Bowman was sent with 30 mounted men, along with some citizens of Kaskaskia, to secure Prairie du Rocher, St. Philippe, and Cahokia.

Hamilton was later joined by a 33-man detachment from the 8th Regiment of Foot, and roughly 200 Miami, Potawatomi, and Shawnee warriors accompanied by British Indian Department officers.

[40] In late January, Clark learned about Hamilton's reoccupation of Vincennes from Francis Vigo, a fur trader and Spanish subject who had briefly been held prisoner by the British.

Clark decided that he needed to launch a surprise winter attack on Vincennes before Hamilton could retake the Illinois country in the spring.

Clark sent the man with a letter to the inhabitants of Vincennes, warning them that he was just about to arrive with an army, and that everyone should stay in their homes unless they wanted to be considered an enemy.

[48] Hamilton initially believed that the gunfire was caused by a "drunken frolic of the inhabitants" but after a sergeant was slightly wounded realized the fort was under attack and ordered his men to return fire.

"[54] In the years that followed, Clark made several attempts to organize a campaign against Detroit, but each time the expedition was called off because of insufficient men and supplies.

Some of the major contributors, such as Father Gibault, François Riday Busseron, Charles Gratiot, and Francis Vigo, would never receive payment during their lifetime, and would be reduced to poverty.

In 1789, Clark wrote an account of the Illinois campaign at the request of John Brown and other members of the United States Congress, who were then deliberating how to administer the Northwest Territory.

The Memoir formed the basis of two popular novels: Alice of Old Vincennes (1900) by Maurice Thompson, and The Crossing (1904) by American novelist Winston Churchill.

In July 1783, Governor Benjamin Harrison of Virginia thanked Clark for "wresting so great and valuable a territory out of the hands of the British Enemy.

In the 20th century, however, many historians began to doubt that interpretation, arguing that because resource shortages compelled Clark to recall his troops from the Illinois Country before the end of the war, and because the Indigenous tribes of the area were unaffected, there was no "conquest" of the Northwest.