America when the Second Amendment was adopted was a small nation on the edge of a vast unsettled frontier that was without government or rule of law. The nation had no professional police forces, and no standing, professional armies.
As for the bit about militias: Even in colonial times, an all-volunteer Army couldn’t beat a professional, standing army. George Washington himself complained relentlessly about that.
How America built itself on guns, then couldn’t let go
[Adam Chang/Vox]