This paper explores what armed forces can do, should do, must do, and should not do, regarding border security and domestic contingencies. It provides historical examples to demonstrate the twin phenomenon of the militarization of civilian functions and the civilianization of military support in multiple countries. It argues these trends are increasing due to limited resources, public will, efficacy, and to new emerging threats including cyber. It also demonstrates how these issues are relevant to border security and migrant operations. The paper delineates and examines the six basic functions of defense support: a) emergency and disaster relief, b) law enforcement, c) special security events, d) essential services, e) counterinsurgency, and f) civil disturbances. It then concludes by forwarding a set of criteria for policy and military leaders to employ when determining whether and to what extent to use armed forces in defense support of these six functions. They include legality, lethality, risk, readiness, cost, appropriateness, capacity, and unique capability.