It’s a bit of both. In the 18th century, it was abundantly obvious a country needed to be able to defend itself against both foreign powers and internal threats. But it was also very clear that if you paid a group of people to be professional soldiers, you basically always lived under the threat of those people going “That’s a nice country/state you have there” and launching a coup.

Hence the well-regulated militia, because then you don’t need a proffesional military, and there’s nobody to launch a coup, and also no way for the federal government to take over individual states. So in a way, it WAS to prevent government overreach, but not in the wat it’s usually said.