Defense Primer: Department of the Army and Army Command Structure [Updated January 2, 2020] [open pdf - 488KB]
From the Overview: "Article I, Section 8, Clause 12 of the Constitution stipulates, 'The Congress shall have power ... to raise and support Armies ... make rules for the government and regulation of the land and naval forces ... for calling forth the militia to execute the laws of the Union, suppress insurrections and repel invasions.' The Department of the Army (DA) is one of the three military departments reporting to the Department of Defense (DOD). The Army's primary mission is to fight and win the nation's ground wars. The Army's mission is both operational and institutional, and it is composed of four distinct components: the regular Active component (AC), the reserve components of the United States Army Reserve (USAR), the Army National Guard (ARNG), and Department of the Army civilians (DAC)."
CRS In Focus, IF10544
Congressional Research Service: https://crsreports.congress.gov/