The siege of Fort Vincennes, also known as the siege of Fort Sackville and the Battle of Vincennes, was a Revolutionary War frontier battle fought in present-day Vincennes, Indiana won by a militia led by American commander George Rogers Clark over a British garrison led by Lieutenant Governor Henry Hamilton. Roughly half of Clark's militia were Canadien volunteers sympathetic to the American cause. After a daring wintertime march, the small American force was able to force the British to surrender the fort and in a larger frame the Illinois territory.