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 officers, ahead of the invasion, disguised themselves as civilians to observe the Grebbeline's potential threats and opportunities. The Rhenen area, near the Rhine river, was particularly noteworthy due to its proximity to German soil and perceived weakness in Dutch defenses.