Just a historical sidelight, which may give a little wider meaning to that Constitutional provision:

The United States after the Revolution, either under the Articles of Confederation or under the new Constitution, neither had nor expected to have a standing army, or at least a standing army of any significant size at all. Just as with the old collection of 13 colonies/states, there was much too much rivalry and independence. The outlook of many was that the central government was to be more or less a weak coordinator to the thirteen colonies/states, rather than a central government to the entire country. It was to be to the United States more or less what the UN is to the world today--underfunded, nearly powerless, and ineffectual.

The Continentals had a lot of experience in colonial times with militias, and they had proved very helpful by themselves against the British, and then as sources of military manpower when the Congress formed an actual army. When a central army was raised, it was intended to be for temporary existence and for use only in a specific emergency which needed to be addressed.

So, given the experience of militias in then-living memory, although there were significant problems with that system, it was felt necessary to provide militias for the functions now served by the Army, the Army Reserve, and the National Guard. For that purpose it was necessary to form informal, local units which might provide cadres of at least partially trained soldiers against a time of sudden need.

Dave Oesterreich