The Netherlands had joined the war because they feared a French annexation of them as they had annexed France as well as the fact they wished unification with Flanders and view the popular nationalists as evil.
So as Germany declared war they did at the same time and started mobilizing their forces and planned to deploy them to the front. Most would not get the chance.
France and Britain mobilized at the same but British standing forces planned to win a quick propaganda victory and the I Corps consisting of 4 divisions had been stationed along with a fleet at Great Yarmouth in preparation for a quick attack. The attack plan had been prepared in 1932 as the Jersey pact was formed and updated in 1936 as tensions rose at the start of the Spanish Civil War.
The fleet set off on the 9th January only 12 hours after the start of the war. The fleet consisting of 3 battleships, escorts and transports sailed from Great Yarmourth to IJmuiden in just 18 hours.
Surprise was total with almost no defenders actually stationed in the city and a British transport sailed right into the harbour as the battleship Redbourn made a radio broadcast to the people of the city demanding their surrender or be killed. Much of the city simply did surrender. The first British injury of the war was when a man fell in the sea and pneumonia. Overall there were only 11 British casualties in this attack on the city and only 6 of them were from Royal Netherlands forces.
The transports then piled out their troops as fast as they could and in 24 hours 4 divisions of the I Corps were in the city and J. F. C Fuller gave the order to march on Amsterdam. The British while could have been slowed down easily by the simple blowing of bridges no one had expected the British to arrive so near Amsterdam that no explosive charges had been prepared for the destruction of bridges.
The only opposition on the way to Amsterdam was a division of infantry at Spaarndam which were strafed heavily by aircraft in Britain this force was batted out of the way when the 1st Armoured Division simply drove straight into them and they were routed roughly 300 British men were killed and it delayed them by an hour.
As they marched through Amsterdam few people could simply understand how they had been so badly defeated so quickly. Though Amsterdam was captured the government refused to surrender unconditionally believing the Germans would come save them yet on the 12th January while the capital was taken Rotterdam was fire bombed by the Axis causing the monarchy to hand itself over and unconditionally surrender.
This deeply shook the Socialist powers across the world as they feared how quickly the Axis had won the battle.