First American Regiment The First American Regiment was the first peacetime regular army force authorized by United States Congress after the American Revolutionary War . It served primarily on the early Western frontier of the United States . Formation . After the conclusion of the American Revolution in 1783 , Congress ordered the Continental Army to disband , and General George Washington resigned his commission as commander - in - chief . The disband order was never carried out , and on 2 June 1784 , Congress reissued the order , under the principle that " standing armies in time of peace are inconsistent with the principles of republican government , dangerous to the liberties of a free people , and generally converted into destructive engines for establishing despotism . " On 3 June 1784 , Congress passed a new resolution : Resolved , That the Secretary at War take order for forming the said troops when assembled , into one regiment , to consist of eight companies of infantry , and two of artillery , arming and equipping them in a soldier - like manner : and that he be authorised to direct their destination and operations , subject to the order of Congress , and of the Committee of the states in the recess of Congress . Thomas Mifflin , the president of Congress , named his former aide , Josiah Harmar , to be the commander of the new regiment , with the rank of Lieutenant Colonel . The new regiment was used primarily to man frontier outposts and guard against Native American attacks . An Artillery Company of the New Regiment was Captain John Doughty ' s Company of the 2d Continental Artillery Regiment which had been attached to the 1st American Regiment ( 1783 - 1784 ) . In 1786 , Secretary of War Henry Knox ordered Col . Harmar to the outpost village of Vincennes to drive away the Kentucky militia , who fled at the approach of the First American Regiment . Col . Harmar left 100 regulars under the command of Major Jean François Hamtramck to build a new fort and conduct operations deep within Indian Territory . Northwest Indian War . The First American Regiment was renamed the Regiment of Infantry in 29 September 1789 . In 1790 , Harmar – now a general – led 320 regulars and over 1 , 000 militia on the disastrous Harmar Campaign . The Regiment of Infantry suffered over 70 casualties . The following year , 1791 , the regiment was redesignated as the 1st Infantry . It comprised the main force of regulars under General Arthur St . Clair in the campaign to destroy Kekionga , a large Miami village central to the Native American forces in the Northwest Indian War . While on campaign , the 1st Infantry under Major Hamtramck was sent to find an overdue supply train . The supplies were never found , but as the Infantry marched to rejoin the main force on 4 November 1791 , gunfire was heard . Native Americans under Miami Chief Little Turtle had attacked in what came to be known as St . Clair ' s Defeat , the worst loss to American Indians by the United States Army in history . Survivors broke through Little Turtle ' s lines and warned Major Hamtramck , who fell back to Fort Jefferson instead of covering the retreat . The next year , 1792 , the First American Regiment was again renamed , and became the Infantry of the 1st Sub - Legion , commanded by Hamtramck . This new Legion of the United States , under General Anthony Wayne , trained extensively before marching North to meet the Native American Confederacy that had defeated St . Clair . The Legion was victorious at the Battle of Fallen Timbers in 1794 . The trading post at Kekionga was rebuilt and named Fort Wayne , and was garrisoned by the First Sub - Legion under Major Hamtramck . William Henry Harrison , later to become President of the United States , was a junior officer in the 1st Sub - Legion . Heritage . In October 1796 , the 1st Sub - Legion was again renamed the 1st Infantry . In October 1815 , the 1st Infantry was consolidated with the 5th , 17th , 19th , and the 28th Infantry to form the 3rd US Infantry Regiment ( The Old Guard ) . The 3rd Infantry , with their roots in the First American Regiment , is recognized as the oldest regular regiment in the United States Army .