system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did countries control their borders hundreds of years ago?
gpt
Patrolling militaries.

You had a lot of natural borders such as rivers, mountain ranges, ridges, forests, etc. And the travel points over, through and between each of these would be garrisoned with a fort and soldiers.

These soldiers would control who goes in and out and keep the borders.

If this fort was taken by an enemy, then that entire area would become the enemies and could be a staging ground for a major invasion.