The snarky answer is that it’s a militia in the sense of “There’s your Second Amendment, now let’s ban guns from the general population”.
The more reasoned answer is that it is a militia- sorta -kinda -in the same sense that Roderic de Borgia a.k.a. Pope Alexander VI was the successor to the Apostle Peter. Someone could write a whole book on the history of gun rights and law in the USA, starting from English antecedents and up to the present day, but to try to condense it to a few relevent paragraphs:
The original Militia Act of 1792 had required all able-bodied men not otherwise disqualified to keep their own service muskets and ammunition. But unlike the Swiss model where the entire male population had to undergo military training, this was thought impractical and unnecessarily burdensome. The requirement was reduced to participating in a yearly muster to demonstrate their readiness. But over the years even that practice became moribund. While in principle all able-bodied men in good standing were theoretically members of the militia, in practice the states routinely relied on a fairly small core of volunteers who formed each state’s standing militia.
During the American Civil War, the Union government had the authority to call out the Militias to help in suppressing the rebellion, including conscripting any able-bodied man. But by precedents going back to feudal England it was held that they could only be held to short-term service, ususally nine months, and that if they could they could pay a substitute to take their place. During the war the Union spent millions of dollars on signing bonuses to get recruits to voluntarily enlist for three-year terms in the regular Army. (The Confederacy by contrast simply asserted a general authority to conscript men for the duration.)
What eventually led to the formation of the modern National Guard was the Spanish-American war. The American forces consisted of regular Army Troops and volunteer corps recruited from the state militias. The quality and preparedness of the militia volunteers was found to be poor, and after the war there was an impetus for providing that all state militias should be trained to a Federal standard and structured to as to be able to be integrated with the regular army during wartime. The Militia Act of 1903 carried out these reforms under Congress’s power to regulate the state militias given in Article One Section Eight, clauses 15 and 16 of the Constitution. It defines trained and actively serving volunteers as the “organized” militia and everybody else as the “unorganized” militia.
Additionally, during World War One the Supreme Court, wich rather jingoistically handed down several rulings against any legal challenges in opposition to the war, ruled that Congress’s power to declare war included an implied power to draft people into the Federal Army (something that would have astonished Abraham Lincoln).
So now the role of the states and the Federal government with respect to military service have been reversed 180 degrees. Formerly only states could require citizens to undergo armed civic duty, while the Federal government could only enlist volunteers. Now the Federal government can draft at will and I can’t even think of a time in recent history when someone was involuntarily deputized to armed duty at the state level.
So the National Guards are sorta kinda a militia of volunteers recruited at the state level. By law and by practice, the National Guards of the various states are tightly knitted with the US Army Reserve. The Federal authority over the guard was even further enhanced when the Supreme Court ruled that state governors cannot forbid their troops from serving overseas in “training” missions.
