A german invasion of Italy would be a very hard thing indeed. The only access points are a small section on France's south, or from Austria. Both of which are mainly moutain terrain. WWI showed what figthing there would mean. Italy would be on the defensive (and I'd wonder what forts they sill have in the area) so it's relative lack of preparedness would be less of a problem, and air support (a german strong asset) would be nearly useless.