Originally Posted by Ikari
That is incorrect; the Supreme Court has specifically said the militia referred to in Article 1 and the 2nd Amendment are the same thing. They further said that the organized militia is a subset of the larger militia (the pool of people that can be called up).
The Army National Guard website also says that is exactly what it is:
About the National Guard
The National Guard, the oldest component of the Armed Forces of the United States and one of the nation's longest-enduring institutions, celebrated its 370th birthday on December 13, 2006. The National Guard traces its history back to the earliest English colonies in North America. Responsible for their own defense, the colonists drew on English military tradition and organized their able-bodied male citizens into militias.
The colonial militias protected their fellow citizens from Indian attack, foreign invaders, and later helped to win the Revolutionary War. Following independence, the authors of the Constitution empowered Congress to "provide for organizing, arming, and disciplining the militia." However, recognizing the militia's state role, the Founding Fathers reserved the appointment of officers and training of the militia to the states. Today's National Guard still remains a dual state-Federal force.
The National Guard - About the National Guard
It is directly derived from Article 1 of the Constitution, and is the same thing as the Militia referred to in the 2nd amendment, per the Supreme Court.