Defense Primer: Military Use of the Electromagnetic Spectrum [Updated September 27, 2021] [open pdf - 563KB]
From the Document: "Since the introduction of the two-way radio in the early 1900s, militaries have been interested in the electromagnetic spectrum ('the spectrum'). The proliferation of spectrum-dependent systems in all military domains--air, land, sea, space, and cyberspace--along with Department of Defense (DOD) concepts, such as net-centric warfare and multidomain battle, increases the military's dependence on the spectrum."
CRS In Focus, IF11155
Congressional Research Service: https://crsreports.congress.gov/