In the months between the Invasion of Poland and the start of "Fall Gelb" (German invasion of the Low Countries), the Dutch military attaché in Berlin, Major Bert Sas, was warned by German Abwehr-officer Col. Hans Oster about the data of the German invasion of the Netherlands. However, because Hitler kept postponing the date, Sas kept warning the Dutch Government regularly between 12 November 1939 and 9 May 1940. When the actual and final warning came, on 9 May, the Dutch Governement was so fed up with this they simply didn't believe him. That night, the Germans invaded. Most defences were not prepared.

The Dutch defenders against the German invaders used mounted cavalry against German troops in Groningen. The Dutch also pulled antique pieces of artillery out of the War Museum to use against the Germans.