What came first the active duty or the reserve?

National Guard - 13 December 1636
Army - 14 June 1775
Navy - 13 October 1775
Marine Corps - 10 November 1775
Coast Guard - 04 August 1790
Army Reserve - 23 April 1908
Navy Reserve - 03 March 1915
Marine Corps Reserve - 29 August 1916
Coast Guard Auxiliary - 23 June 1939 (Civilian Auxiliary)
Coast Guard Reserve - 19 February 1941
Civil Air Patrol - 01 December 1941 (Civilian Auxiliary)
Air Force - 18 September 1947
Air Force Reserve - 14 April 1948
Space Force - 20 December 2019

Concepts for a Space Force Reserve and Space National Guard are currently being debated in Congress.

The Reserves is the youngest of the 3 Army components, having been established on 23 April 1908

For the US Army, the National Guard came first. By about 100+ years or so.