Germany invaded France, why did the Netherlands defend the German army with a flood ?

Because the Netherlands is a low-lying country, a large part of the country's land is "reclaimed from the sea" polders. As a result, part of the country is below sea level and will be submerged if it is hit by floods, so the Netherlands has had a flood prevention policy for several years. So they defended the Germans with floods.

Because the German tank armor was particularly strong when the Germans invaded France, if the Netherlands did not use tanks against the tanks, it would cause a large number of soldiers**.

Topography reasons. The Dutch had limited troops, but the "Dutch Fortress" had "flood defense", and once the floodgates were opened, part of the country would be submerged.

Because more than 25 percent of the Netherlands is located below the surface of the sea, when the floodgates are opened and the water is released, it can be used as an effective means to stop the enemy's aggression and attack.

Because a large part of the Netherlands is "reclaimed from the sea", 25% of the country's land is located below the mean sea level, so it can be flooded by opening the floodgates and releasing water, so as to resist foreign invasion.

The Dutch had limited forces and were not strong enough to defend the 400-kilometer border from Maastricht to the North Sea. In order to defend against the German invasion, they had three lines of defense: they had built general fortified positions in the border area, and only a small number of troops were deployed; Then there was the Gréby-Peel Line, on which the 10 Dutch infantry divisions relied to organize their defenses; Finally, there is the "Dutch Fortress", the region of Rotterdam, Amsterdam, Utrecht and The Hague, which has bays, rivers and large areas of water that constitute good natural barriers, and is surrounded by the Grave fortification area to the east with the IJssel Canal to the north, and the fortifications from the Waal River to Rotterdam to the south, and the "Dutch Fortress" is the seat of the central nervous system of the Netherlands.

In order to delay the Gréby-Peel Line as long as possible with the Germans, the dams on the Lower Rhine, Maas, and Vaal rivers could be opened if necessary, so that large amounts of water could form an obstacle in the area and facilitate the defense of the main port cities.

The Germans were clear about the possibility that the Netherlands might use the water barrier for defense. At that time, there was a way to dismantle the plan of the Dutch army to avoid the threat of flooding for the German armored forces: while the ground forces broke through the main line of defense, they simultaneously attacked and demolished the fortified bridges over the three main rivers to ensure the rapid passage of the German army.

This is the main task of airborne operations.