To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;

To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress

source https://constitution.congress.gov/constitution/

Actually, the "militia" they were referring to is the fighting force composed of all the armed, able-bodied men in the country. The Militia is formed when these armed civilians are called into service.

This is why the 2A actually implies that every one of us should be armed with military-grade firearms. Together, we are the Continental Army.