Today, 14 June, marks the 248th birthday of the US Army. On that date in 1775, the Continental Congress authorized ten companies of expert riflemen. That makes the Army the senior service predating the Navy (13 OCT 1775) and the Marine Corps (10 NOV 1775).
The oldest Regular Army infantry regiment, the 3d Infantry Regiment, was constituted on 3 June 1784, as the First American Regiment.
The Army has participated in every American conflict from the Revolutionary War to the Civil War to the World Wars to the Global War on Terror and many more in between.