Way ahead of the actual invasion German army staff officers managed to visit the Grebbeline in civilian outfits, carefully studying the actual threats and opportunities. Particularly the Rhenen area, close to the Rhine river, was noticed. It was the shortest away from German soil and seemed to be a weak spot in the Dutch defence line.
German military strategists, anticipating an invasion, disguised themselves as civilians to observe the Grebbeline's strategic significance, particularly the Rhenen region, which presented a potential weakness in Dutch defenses.