In January 1778, Lieutenant Colonel George Rogers Clark of the Kentucky militia was authorized by Virginia Governor Patrick Henry to lead an expedition to seize the British outposts of Kaskaskia and Cahokia on the Mississippi River.

The decrepit stockade of Fort Sackville, located in Vincennes on the Wabash River 180 miles east of Kaskakia, was occupied by the end of the month by a small detachment led by Captain Leonard Helm.

He 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.

Vigo provided Clarke with information about the strength of Fort Sackville's garrison and Hamilton's plan to retake Kaskaskia and Cahokia in the spring.

He explained his reasons in a letter to Governor Henry: I know the case is desperate; but, sir, we must either quit the country or attack Mr. Hamilton.

Later, in a letter to his friend and mentor George Mason, Clark described his feeling for the journey as one of "forlorn hope," as his men were faced with a long trek over land that was "in many parts flowing with water.

"[5] The day after Clark and Bowman began their march, 40 men under Captain John Rogers departed Kaskaskia in the armed keelboat Willing.

Clark later recalled that the weather was "wet, but, fortunately, not cold for the season" but found "a great part of the plains under water several inches deep."

They reached the Little Wabash River on February 13 and found it had burst its banks, flooding an area about 5 miles (8.0 km) wide.

"[5] Even so, a February 20 entry in Captain Bowman's journal describes the men in camp as "very quiet but hungry; some almost in despair; many of the creole volunteers talking of returning."

Shortly before reaching Vincennes, another hunter was captured who informed Clark that the British remained unaware of the American presence.

While most of Clark's men secured the village, small detachments stealthily approached the fort, took cover behind nearby fences and buildings and opened fire.

He 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.

A scouting party that Hamilton had sent out on February 22 to investigate the report of campfires was able to infiltrate through enemy lines to the fort, however, two were captured.

Although the British regulars enthusiastically supported their commander's decision, the militia who were inside the fort were unwilling to continue fighting since many of their relations were openly assisting Clark.

With seven of his regulars wounded, little hope for reinforcement, and a militia who he later called "treacherous cowards," Hamilton sent out a letter offering terms; asking for a three-day truce and a meeting with Clark.

After Hamilton indicated his willingness to fight it out, a compromise was reached wherein the British would "deliver themselves up Prisoners of War and March out with their Arms and Acoutriments.

[9] Hamilton was treated as a criminal rather than a prisoner of war due to false claims that he paid Britain's Indigenous allies for scalps.

[10][11] William Hayden English, among others, attributed Britain's ceding of the territory northwest of the Ohio Country in the Treaty of Paris to Clark's exploits.

Clark repeatedly attempted to organize campaigns against Detroit, but even with the support of Virginia's governor he was never given sufficient men and supplies.

[10] Following Clark's victory, the Virginia General Assembly gave official status to the region northwest of the Ohio River and named it Illinois County.