The 2nd amendment was drafted at a time when there were no standing armies. While cannons existed, most battles were decided by individual soldiers with their personal weapons.

In the modern world, you still need individual soldiers with individual weapons to hold terrain, but it's very difficult to advance or take terrain from a modern military without artillery and bombs. So, a militia could maybe ensure that an attacking force was never able to fully settle and control a region (i.e. what the Taliban managed vs the western forces in Afghanistan) it can't really conquer much if opposed by a real army with real army toys.

The real joke of the 2nd amendment is that it was obviously intended to be about citizen militias used to fight off opposing armies, but has been transformed into people's right to own whatever they want for "self defence", which 90% of the time actually means suicide, road rage, domestic violence, accidental shootings, mass shootings, gang warfare, police shootings, armed robberies, etc.