Yes it's complicated and I've never had to study it in detail. Of course you're not talking about one country - but at least Greece
, many of whom have fought centuries of wars, so keeping the peace has been a tortuous process.
isn't that easy, going that other way, where as a Brit not wanting to work or pay Aus tax, I can only get a tourist visa:
An Australian tourist visa (ETA) will allow you to enter Australia for the purpose of tourism. This visa is valid for 12 months from the date of issue, and allows multiple entry into Australia.
An ETA will allow you to stay in Australia for up to 3 months on each visit.
So yeah, I can stay up to a year but only 90 days at a time, and where the hell am I going to go to restart my time?
And the US is even more complicated.