Alexander Hamilton explains his definition of what "well-regulated" is - and the purpose of a citizen militia - in contrast to the standing army in the Federalist Papers, No. 29. Most of the idea has become much more federalized than intended with the National Guard, but it has long since been misused for its intended purpose.

A bunch of farmers that go shooting drunk. /s

Seriously though, everyone back in the 1700s realized that all Americans were American. I'm not sure that's true any more.