Fort McIntosh was an early American log frontier fort situated near the confluence of the Ohio River and the Beaver River in what is now Beaver, Pennsylvania. The fortress was constructed in 1778 under the direction of Lt. Col. Cambray-Digny, a French engineer, and named in honor of General Lachlan McIntosh. The fortress was the site of the signing of the Treaty of Fort McIntosh on January 21, 1785. It was occupied until it was abandoned in 1791. After the Revolution, the fort was the home of the First American Regiment, the oldest active unit in the United States Army.