German invasion of the Netherlands
During the plans for Fall Gelb the idea of leaving the Fortress Holland alone was considered. The first plan of 19 October 1939 suggested a full occupation. In the version of 29 October it was proposed to limit the invasion to a line south of Venlo. In the Holland-Weisung (Holland Directive) of 15 November it was decided to conquer the entire south, but in the north to advance no further than the Grebbe Line, and to occupy the Frisian Isles.