There are several historical processes that can lead to relative deprivation. Among them is rapid deterioration in material living conditions, which may occur for the whole population of a country during an economic depression or for only some population groups during periods of transition in the economic system. A wide breach opens between the way people expect they should be able to live and their capabilities of meeting those expectations -- not because of a change in expectations, but because of a decline in capabilities of attaining them. This type of relative deprivation may also result when one country is invaded and conquered by another. The victor nation may exploit the resources and labor power of a defeated people, and a drastic decline in the standard of living ensues. People in the defeated nation may try to resist occupation forces with violence. During World War 2 many of the peoples whose nations were invaded by Nazi German or Japanese forces organized resistance groups, which, in some instances, grew into revolutionary movements; the latter not only helped expel the invaders but also brought about drastic changes social, economic, and political institutions after the war.