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.
The first war tax stamp was produced in 1874 in Spain, during the Third Carlist War. Several more types were issued through 1879, superficially similar to the regular stamps of the time, but inscribed "Impuesto de Guerra" or "Impto de Guerra".
Canada went a step further and modified the dies of its Admiral series of stamps starting in 1915, first to read "WAR TAX", and then in 1916 to read "1 T C", meaning that the stamp carried both its face value and the tax of 1 cent, conceptually similar to a semi-postal stamp, but whose use was required rather than optional.