Immediately following the Polish invasion by the German Army on 1st September 1939, France and England declared war on Germany. France and England then began their mobilisation. General mobilisation was declared in France on 2 September 1939. Within the space of several days, thanks to military service, the reserve armed forces and territorial army, France managed to draft in some five million soldiers, including two and a half million front line troops.
In England, the military call up was voted on 3 September. What had previously been an army of volunteers, began compulsory army service. Between September and January, 1.5 million men were drafted into the army. Belgium and the Netherlands remained neutral at the beginning of fighting. They began their general mobilisation before the invasion of Poland, notably to ensure their territorial integrity.
In Belgium, over 700,000 people joined the army, which was 8% of the Belgian population. The Netherlands began their general mobilisation on 24 August. They managed to group together circa 300,000 men.