The Battle of Vincennes was the only full scale battle to take place in the Middle West during the American Revolution. If the river routes such as the Mississippi and the Ohio were not in American control, then the Brittish could have attacked the American colonies from the rear (so to speak). George Rogers Clark protected America's back door, and the major town of Kaskaskia and Vincennes were the keys to keeping the midwest under american control.

General Hamilton intended to use Vincennes as a jumping off point to launch an attack on the Viriginia colonies and push the Americans back towards the Atlantic Coast. Had he succeeded the war would certainly have lasted longer then it did and might have resulted in a British victory. Although France was already an ally by february of 1779, Spain was not. Spain joined in june of 1779, probably in part because of Clark's interactions with both the spanish and french in the middle west (St. Louis, Kaskaskia, Vincennes, etc.). Spain provided much needed finanical and international support for the young nation and without that money and support the French might have bailed out before the tide turned in 1780.

Regardless of the Battle's importance to the winning of the American Revolution, the Battle itself is an important example in psychological warfare. Clark did the impossible when he marched across Illinois in the dead of winter -- wading several miles in watery swamps and across the Wabash river -- and surprised Hamiliton who did not expect any attack until the spring. Clark had only 127 volunteers with him (half of which were French), compared to at least 300 that Hamilton had. If Clark had waited to attack in the spring he might have had as many as 300, but by then Hamilton would have gotten the british reinforcements from Fort Detroit and would have had 2000-3000 troops. So clark, with 127 men, faced off a well armed fort with a British force that outnumbered him two to one. Clark tricked Hamilton into thinking he had far more troops then he did by having his troops march through the surrouding woods with 24 different flags. Each flag supposed represented a unit of troops that Hamilton assumed (as was battlefield protocol in the day) that each unit was at least 100 soldiers. So Hamilton thought he was greatly outnumbered by Clark's superior force and ordered the troops to protect the fort and not go on the offensive. Hamilton surrendered still believing his troops were outnumbered. Clark could not have taken Vincennes without using daring initiative and psychological warfare.

Hopefully I've explained this clearly and have not confused anyone. My sources mostly include the following books: George Rogers Clark and the War in the West by Lowell H. Harrison, George Rogers Clark and the Winning of the Old Northwest by Robert C. Alberts, George Rogers Clark by Walter Havighurst, and by talking extensively with the staff of the George Rogers Clark National Memorial in Vincennes, Indiana for a mini-docutmentary I made for a local Illinois PBS station.