Also, the countries being passed through obviously cannot be hostile to you.
The consequences of this from a practical perspective as Germany are as follows when usin a somewhat historical pattern. Between declaration against Poland and the fall of France, Germany will suffer a period where their Tungsten imports from Spain and or Portugal can be interdicted. Once France falls both of these are safe, unless you invade Spain at which point, until they surrender your convoys will once again be vulnerable. What is frustrating here is that your route will still head to Wilhelmshaven via the Channel instead of going to one of the French ports along the Bay of Biscay.
For the same reason, any import of tungsten from Sweden is always vulnerable unless the opposition allows Germany to control the Eastern North Sea or puts little effort into blockading Germany. The route from Sweden foolishly passes into the North Sea on its way to Wilhelmshaven instead of going to one of the relatively safe destinations along the Baltic coast.