The well-regulated militia's purpose was to battle and put down a slave and Indian uprisings. It wasn't meant to replace the standing army of united states in battle against foreign invaders . The US didn't even have a standing army after the defeat of Cornwall. The 2nd Amendment was included for slave-holding states to protect the landed slave owning gentry and protect people confiscating Indian lands.

"To oblige the great body of the yeomanry, and of the other classes of the citizens, to be under arms for the purpose of going through military exercises and evolutions, as often as might be necessary to acquire the degree of perfection which would entitle them to the character of a well-regulated militia"

Of course this efficiency refers to the militias, and rightly so - the people serving in the militias had to be effective in that role. Being well armed and well trained was imperative to defending their/our freedoms.

“The Congress shall have Power To ...provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions....”

Article I, Section 8, Clause 15


Militia Act 1792

Section 1. Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled

"That whenever the United States shall be invaded, or be in imminent danger of invasion from any foreign nation or Indian tribe, it shall be lawful for the President of the United States, to call forth such number of the militia of the state or states most convenient to the place of danger or scene of action as he may judge necessary to repel such invasion, and to issue his orders for that purpose, to such officer or officers of the militia as he shall think proper; and in case of an insurrection in any state, against the government thereof, it shall be lawful for the President of the United States, on application of the legislature of such state, or of the executive (when the legislature cannot be convened) to call forth such number of the militia of any other state or states, as may be applied for, or as he may judge sufficient to suppress such insurrection.

That whenever the laws of the United States shall be opposed or the execution thereof obstructed, in any state, by combinations too powerful to be suppressed by the ordinary course of judicial proceedings, or by the powers vested in the marshals by this act, the same being notified to the President of the United States, by an associate justice or the district judge, it shall be lawful for the President of the United States to call forth the militia of such state to suppress such combinations, and to cause the laws to be duly executed.