In the Miller decision the SCOTUS defined the “Well-Regulated Militia” of the 2cd amendment as “,…all males physically capable of acting in concert for the common defense.” (United States v. Miller, 307 U. S. 174, 179 (1939)).
The Heller decision reaffirmed that above and quoted several founding era (the time around the formation of the United States) sources that agree with this definition (District of Columbia Et Al. v. Heller, 554 U. S. 22 (2008))
The Militia Act passed by the first Congress of the U.S. in 1794 stated the Militia was all able bodied males between 18 and 45. It also made distinctions between the "unorganized" and Militias called up for service by the State of Federal Governments. The Texas State Guard (mentioned above) and the National Guard of today would be to modern day equivalent of Militia called up for duty. (Although some would argue the National Guard is just a branch of the Federal Military.)
Around the time of the Revolution and the decades afterwards some towns and groups got together on a regular basis to drill and train. The “Michigan Militia” and the like may or may not be the modern equivalent, depending on how much time they actually drill vs. drink and brag.
The need for the unorganized militia to be called up for militia duty has been extremely rare in this county over the last 100 years. (The Pancho Villa raids across the border into New Mexico in 1916 & 1917 is one notable exception. Another might be Appalachian coal miners forming armed groups against armed forces raised by the miner owners in the 1920’s.) )
More often the unorganized militia (citizens willing and able to work together for the common good) has been required for non-military emergencies. Like when people turn out to fill sand bags to hold back a rising river, that is the militia. Or the people who showed up by the hundreds out the jail that was holding Martin Luther King, and stayed there night and day to prevent him from being lynched, that is the militia.
