[ijidak]

It's true. Control of many fringe regions was tenuous at times.

For example, the Annals of Sennacherib show the Assyrians had to re-conquer areas that drifted after previous subjugation.

But how do you draw those boundaries between subjugations?

In the Bible, it shows that Nebuchadnezzar's armies had to repeatedly return to Israel to re-subjugate it between rebellions.

No doubt similar rebellions could occur in many different areas all at once.

Boundaries were likely never as neat as the boundaries we show today.

Plus, it's not like they were surveying their boundaries and monitoring them via a supranational body like the U.N.

[digging]

Borders are a more modern concept anyway, and even modern borders are anything but a line on a map. (Consider airports. Embassies. Checkpoints. Federal agencies operating inside neighboring countries to deter migration. Etc)

Boundaries are policies, not objects. They always have been.