See P. C. Jessup, The Early Development of the Law of Contraband of War (1933).
However the term is also commonly and in legal language used for goods that by their nature, e.g. too dangerous or offensive in the eyes of the legislator (those are termed contraband in se) are forbidden, and for so-called derivative contrabande, i.e. goods that may normally be owned but are liable to be seized because they were used in committing an unlawful act and hence begot illegally, such as:
Conditional contraband, formerly known as occasional contraband, consists of such materials as provisions and livestock feed. Cargo of this kind, while presumably innocent in character, is subject to seizure if, in the opinion of the belligerent nation that seizes them, the supplies are destined for the armed forces of the enemy rather than for civilian use and consumption. In former agreements among nations, certain other commodities, including soap, paper, clocks, agricultural machinery and jewelry, have been classified as non-contraband, although these distinctions have proved meaningless in practice.
Under conditions of modern warfare, in which armed conflict has largely become a struggle involving the total populations of the contending powers, virtually all commodities are classified by belligerents as absolute contraband.
Numerous treaties defining contraband have been concluded among nations. In time of war, the nations involved have invariably violated these agreements, formulating their own definitions as the fortunes of war indicated. The Declaration of London, drafted at the London Naval Conference of 1908-1909, and made partly effective by most of the European maritime nations at the outbreak of World War I, established comprehensive classifications of absolute and conditional contraband. As the war developed, the lists of articles in each category were constantly revised by the various belligerents, despite protests by neutral powers engaged in the carrying trade. By 1916 the list of conditional contraband included practically all waterborne cargo. Thereafter, for the duration of World War I, nearly all cargo in transit to an enemy nation was treated as contraband of war by the intercepting belligerent, regardless of the nature of the cargo. A similar policy was inaugurated by the belligerent powers early in World War II.
Under international law, the citizens of neutral nations are entitled to trade, at their own risk, with any or all powers engaged in war. No duty to restrain contraband trade is imposed on the neutral governments, but neither have neutral governments the right to interfere on behalf of citizens whose property is seized by one belligerent while in transit to another. The penalty traditionally imposed by belligerents on neutral carriers engaged in commercial traffic with the enemy consists of confiscation of cargo. By the Declaration of London, this was extended to include condemnation of the carrying vessel provided that more than half the cargo was contraband. The right of warring nations to sink neutral ships transporting contraband is not recognized in international law, but this practice was initiated by Germany in World War I and was often resorted to by the Axis Powers in World War II.