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2.1 Pre-1945 period

The youthfulness of international environmental law is clearly shown by the scarcity of relevant legal instruments prior to the Second World War. Only following the Second World War did the disparate bilateral and regional treaties on international environmental issues start to grow into a more coherent body of international law. The first period has its origin in the bilateral treaties on fishing of the 19th century and ended in 1945 with the creation of the United Nations.

It can be argued that the early agreements were motivated largely by competing economic interests to regulate better limited resources such as fish. Indeed purely economic interests may be viewed as driving struggles for navigation rights on shared river systems. The French Republic proclaimed the principle of free navigation in 1792. Navigation remained a principal basis for international agreements governing shared uses of international rivers until the early 20th century. By the beginning of the 20th century, relevant international environmental agreements concerned three main areas: boundary waters; freshwater fisheries; and the conservation of natural resources. A leading example of an agreement concerning boundary waters is the 1909 USA-Canada Boundary Waters Agreement (BWT 1909). This states that boundary waters and waters flowing across the US-Canadian boundary 'shall not be polluted on either side to the injury of health or property on the other'. The question of apportioning rights over boundary waters shared by the United States and Mexico also led to conventions. For example, the 1889 convention established the International Boundary and Water Commission to administer boundaries and water-rights agreements between the two countries. An agreement in 1906 provided for the equitable distribution of the Rio Grande waters between the USA and Mexico.

Other early boundary water conventions that have environmental provisions include the London Agreement between British and German territories regarding the Yola to Lake Chad, the 1908 Berlin Convention which defined the frontier between the Cameroons and French Congo, and the 1913 Cameroons-Nigerian agreement. In 1911, the Institute of International Law (IIL) issued the Madrid Resolution on international regulations governing the use of watercourses.

With respect to freshwater fisheries, their increasing importance regionally and globally was reflected in many agreements concerning fishing rights and the conservation of fisheries. An early agreement was the Russia-USA, Convention Regulating Navigation, Fishing, Trading and Establishment on the Northwest Coast of America, St Petersburg, 17 April 1824. In 1839, France and Great Britain adopted the Convention for defining the Limits of Exclusive Fishing Rights. Great Britain's economic interest in fisheries was further expressed in the Treaty with the United States Regarding the North Atlantic Fisheries. This was followed by a further agreement between the two countries captured by the Treaty for the Settlement of the Fishery Question on the Atlantic Coast of North America in 1888. Also in Europe, in 1869, the Berne Convention established regulations over fishing in the Rhine between Constance and Basel followed by other agreements. Rumania and Serbia adopted the Bucharest Convention in 1908 concerning fishing in the Danube.

From the beginning of the 20th century there was increasing interest in protecting natural resources in addition to fish. A landmark convention was the 1911 Fur Seals Convention. Lesser-known agreements were a 1900 convention for the protection of wild animals, birds and fish in Africa, and in 1902 there was a convention for the protection of birds useful to agriculture which again had an economic basis.

Examples of such international treaties, codifications attempts and important decisions of international tribunals in the first half of the 20th century are:

The Trail Smelter case (UN 1938) is foundational in international environmental law. In 1896, smelting operations began in Trail in British Columbia in Canada. 30 years later, coincidental to an increase in the smelting, two stacks were built to emit waste gases including sulphur dioxide at a height of 409 feet. This correspondingly caused increased transboundary emissions and consequent damage in Washington State immediately south of British Columbia. In June 1927, the United States Government took up the issue. A referral to the Canada-USA International Joint Commission (IJC) failed to deliver an acceptable outcome. Subsequently, under an agreement reached in 1935, both governments agreed to establish a Tribunal. The decision of the Tribunal was reported in two parts, in 1938 and 1941. The decision established two fundamental principles of liability for transboundary harm. A state must show that there is harm caused by the emission from another state and not simply that a transboundary emission has occurred. The tribunal also affirmed the principle that a state has a duty to prevent polluting activities that cause evident injury in a neighbouring state.

In October 1938, the International Union for the Protection of Nature (IUPN) formed to promote the preservation of wildlife and the natural environment, improvement of wildlife education, scientific research and wildlife legislation. The IUPN was later renamed as the International Union for Conservation of Nature and Natural Resources (IUCN). The IUCN, with its seat in Switzerland, is today one of the most active international non-governmental agencies working on international environmental protection.