> National borders don’t limit power transmission

Oh boy, you are very naive, ain't you? No country will put their energy stability in hands of their neighbors. Or do you see China, Korea, India and Russia interconnect their grids? What other supranational grids you know of?

Russia, China, and most of the rest of the world has interconnected their grid with other countries.

This is a map of where the grid is frequency matched across internal borders allowing easy exchange. DC connections don’t need this synchronization, but it reduces losses with AC. https://en.wikipedia.org/wiki/Continental_Europe_Synchronous...

In war this might be a minor issue, but in war electrical infrastructure ends up a target so it doesn’t matter that much.