War tax stamp

5 centimo war tax stamp of Spain, 1875.
1918 Maltese war tax overprint.
Canada's war tax stamps were produced by die modification.
British Guiana war tax stamp on cover.
North Borneo war tax overprint, 1941.

A war tax stamp is a type of postage stamp added to an envelope in addition to regular postage. It is similar to a postal tax stamp, but the revenue is used to defray the costs of a war; as with other postal taxes, its use is obligatory for some period of time.