Dear Chris,
thank you for your answer. But I think you do not understand me.
I know how I can show some posts from a category, BUT I do not know how I can make, that it goes automatic (I describe it already, but okay):
I want that:
If you read post “Napoli”, the map in sidebar shows all posts in category “italy”;
If you read post “Rome”, the map in sidebar shows all posts in category “italy”;
If you read post “Milan”, the map in sidebar shows all posts in category “italy”;
BUT if you read post “Paris”, the map in sidebar shows all posts in category “france”.
SO I need that it knows automatic which category has the post and shows the right map.
I do not know how I can it explain in other ways…