In the afternoon of May 9th, 1940, a Thursday like today, Hitler and a few members of his staff left the Reichskanzlei in Berlin by car and boarded a train at an inconspicuous site somewhere to the north of the city. The train drove north, and Hitler jokingly promised his secretary a fur coat. Many aboard assumed that a trip to recently invaded Norway had been planned.
However, as the night fell, the train turned west, to reach new headquarters in the Eifel mountains in the early hours of the 10th. And on it’s way there, it stopped at Celle near Hannover, where a phone call to Berlin was made. After a confirmation that auspicious weather was to be expected, the order was given at 9 PM to issue the code word “Danzig” to all Wehrmacht and SS units in the west. This was the signal that Fall Gelb, the attack on the Netherlands, Belgium and France, would start at daybreak.
Earlier that day, back in Berlin, Colonel Hans Oster of the Abwehr informed his close friend Bert Sas, the Dutch military attache, that the orders had been given and unless they were revoked by 9:30 PM, the invasion would start tomorrow morning. Sas immediately informed his Belgian colleague, and then, contrary to his expectations, he managed to contact the Dutch authorities in the Hague by phone and tried to warn them of the imminent danger, but while the overall tension was high, they were not quite ready to believe him. Fall Gelb had been planned and postponed many times before, and each time Oster had warned Sas, and Sas had warned the Dutch government.
This time, he was right.