Why does the arbitrary nature of countries favor the invader though? What about the argument that Russia is an arbitrary country that doesn't need to violently expand into neighbouring countries?

It's actually not bad to watch 13 if you're not already a fan in my experience. Series 11 was the first I ever watched, and it hooked me in with some of the better episodes (Rosa, Punjab, Takes you away) and Jodie's performance was really engaging for me. The overarching story (Tim Shaw) was not good though.

I since went back and watched all of new who, and 13 is still possibly my second favorite doctor performance after 11 (Ponds only... He got a bit weird in the Clara series).