For you and Novus both, the term you are overlooking is "militia," not "well regulated." You need to understand that for the framers, the militia is every male citizen from the ages of 16 to 60. Remember that the Constitution was written over 13 years after the Declaration of Independence and 6 years after the Treaty of Paris ended the Revolution. So, there had been an American Army established for well over a decade prior to the adoption of the Constitution.
The militia is not the army. The militia is every adult citizen. The reason that it exists is to ward off domestic tyranny not to fight foreign invaders. Citizens must be able to own arms sufficient to chastise the tyrannical impulses government, hence the 2nd Amendment.