"The ultimate right to keep and bear arms belongs to the 'the people,' not the 'states.' As the language of the Tenth Amendment shows, these two of course are not identical and when the Constitution means 'states,' it says so. Thus... 'the people' at the core of the Second Amendment are the same 'the people' at the heart of the Preamble and the First Amendment, namely Citizens...Nowadays, it is quite common to speak loosely of the National Guard as 'the state militia,' but...in 1789, when used without any qualifying adjective, 'the militia' referred to all Citizens capable of bearing arms. The militia is identical to 'the people' in the core sense described above."
Professor Akhil Reed Amar