The State of Holland decided to flood the country to impede the Spanish invasion

The Spanish army kept re-conquering the parts of Holland that the Geuzen had liberated.

In desperation, the State of Holland decided to flood most of the country -- Rijnland, Delfland en Schieland -- to impede the Spanish invasion.

It was easy enough to do, and it was effective. The Spanish retreated back to secure supply lines, but it took years for Holland's land to dry out and for the economy to recover.