The Dutch were allowed to remain neutral in WW1 because of the defence lines they invested heavily in during the decades leading up to it. These were made of concrete fortresses that can still be seen everywhere where I live, combined with an infrastructure that allowed to flood large land areas. This project was called the “Hollandse Waterlinie”. These comprehensive defence works were conceived during a time where airforces didn’t exist yet. When WW1 broke out, airforces were still in their infancy and this made the Netherlands quite difficult to take.
It would be very hard for land forces to drive trough the flooded fields defended by hundreds of fortresses.
During WW2 the Germans simply carpet bombed Rotterdam, almost completely flattening the city.
They announced Amsterdam would be next. So the Dutch government ordered the military, that was holding off German troops at heavy losses, to surrender unconditionally. It was that, or eventually be overrun too, but with all major cities annihilated.
The Hollandse Waterlinie was not designed to fend off huge numbers of bombers.
Source: Bart Schreuder (Quora)
Some more photos added:
