Is there a conceivable reason that it can't mean both?
All other sections of the Bill of Rights prohibit government from doing certain things.
Yet we're supposed to believe the the 2nd (alone) is intended to guarantee a right of governments to do something nobody was trying to keep them from doing and anyway quite adequately covered in the text of the Constitution itself!?
"The Congress shall have Power To ... provide for the common defense of ... the United States"
"To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water;
"To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years"
"To provide and maintain a Navy"
"To make Rules for the Government and Regulation of the land and naval Forces"
"To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions"
"To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress"
"The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States"