Occupation stamps are war time issues that are printed by an occupying force for use in the territory of a conquered area. As such, they are usually quite popular in the country that did the occupying, as sort of a jingoistic memory, and quite unpopular in the country that was occupied, reminding them of their weakness and pain. For example, the Japanese issued hundreds of occupation stamps for their military conquests of East Asia during WW II. These stamps are rarely collected by native Chinese, Burmese or Filipino collectors. But they are avidly sought out by native Japanese. Indeed, when dealers list them in their catalogs they are listed under the issuing country (in this case Japan) as that is where the real interest lies.
There is one exception to this rule is the Germans. They collect the WW II Occupation of Germany more avidly than do any of the occupiers and are especially assiduous in collecting the Soviet Zone Occupations. Partly this is because of the seriousness with which the Germans take our hobby. I’ve often thought that a Michel Deutschland Specialized catalog should be one of the objects placed in a space capsule to show aliens the best Earth has to offer (it has hundreds of thousands of listings and is a collaborative work of art, representing the collective effort of millions of hours of specialists study and work). But partly too it is founded on the way Germany has approached the Nazi period and WW II. No doubt springing from the guilt of the WW II experience that produced National Socialism, there is a realistic, hard nosed, no illusions quality about the Germans view of what happened mid century. There have little fondness for the idea of occupying others and no fear of admitting what happened to them. It seems to be the attitude of a nation that can move forward, philatelically and otherwise.