The second amendment exists in two parts.
First, "A well regulated Militia" establishes the existence of the National guard. (Technically, the only legal army in the US. Read the Constitution, the US Army exists illegaly. The constitution forbids a peacetime standing army.)
Second, Establishes the right of the people to keep and bear arms. (The people being the same people in the first amendment, that is, the general population)
A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.
Separating this ONE sentence seems a mistake. The rationale is that the Militia is necessary.
The question is whether or not the armed citizenry MUST be part of a militia. Nothing here
about the national guard at all. Since the state militias/guard are protected by the exclusion
clause, there seems to be no need whatsoever for this to be a part of the amendment. Thus,
one must assume that this refers to other militias.