The distinction is that the US Military is not, generally, permitted to enforce domestic policy. E.g. you won't find them giving out parking tickets. National s

The distinction is that the US Military is not, generally, permitted to enforce domestic policy. E.g. you won't find them giving out parking tickets. National security (or "the common defense") is a pretty big exception to this rule, and of course includes border security - which is an extension of foreign policy.