The Polish resistance movement was formed soon after the German invasion of Poland in September 1939 and quickly grew in response to the brutal methods of the German occupation. Polish resistance had operatives in the urban areas, as well as in the forests (leśni). Throughout the war, the Polish resistance grew in numbers, and increased the scale of its operations, requiring the Germans to devote an increasing amount of resources (personnel, equipment and time) to deal with the partisan threat.[citation needed]