Invasion and conquest are a Biblical means of judging the invaded nation. The 10 northern tribes were conquered by the Assyrians (child sacrifice + they never worshiped the God of their fathers after the split) and Judah was invaded by Babylon for same reason (70 years).
When they returned the first thing they did WAS BUILD A WALL. :)
I tried to establish that people always had their own land without putting up a boundary.
Besides perhaps a gate and small guardhouse on a road, what determined one country from another ?
I live about seven miles from the West Virginia border and the only indication I have made a transition is a smallish 2' X 3' sign and the condition of the road. Take away the sign and it's one township to another in ANY state.