The District of Columbia has been occupied by a belligerent force retaining prisoners of war, hence the black POW flag flying under the old glory above both the White House and the US Capitol in DC.
11.3 in The Law of War Manual explains what might be termed belligerent occupation is carefully analyzed and Chapter 11 goes on to address when military occupation law applies, how a situation of occupation ends and when GC obligations cease to apply, the authority and obligations of the occupying power e.g.
In relation to legislation and courts, the movement of persons in occupied territory and the protection of children there.