DoD Instruction 7000.14-R
(https://comptroller.defense.gov/Portals/45/documents/fmr/Vol... is a good start)
Overseas Contingency Operations - https://www.cbo.gov/system/files?file=2018-10/54219-oco_spen...
Nonappropriated funds (MWR, etc.) - https://comptroller.defense.gov/Portals/45/documents/fmr/arc...
Foreign military sales / FMS - https://en.wikipedia.org/wiki/Foreign_Military_Sales.