 With the German invasion of Poland in September 1939 , although still following a policy of neutrality , the Belgian government began general mobilization . By 1940 , the army numbered between 600 @,@ 000 and 650 @,@ 000 men ( nearly 20 percent of the male population of Belgium ) making it approximately four times larger than the British Expeditionary Force and twice as large as the Dutch army at the time . 
