How much red tape is there if one wants to cross an international border?
Generally, not much. You need to file a flight plan, which is essentially just telling the ATC where you're going, and where you're coming from. The flight plan is also useful in case you get into trouble, since it contains the identifying information for your plane, the number of people on board, and for instance the safety equipment on board. So if you crash, the search-and-rescue people know what to expect. Depend…
Other than Mathias Rust?