Delevingne was an expert on the control of narcotic drugs. He believed that the key to narcotics control lay in curbing their supply and that the drug trade was "a great evil which must be fought." He convinced his colleagues that growers and manufacturers must be forced to cut back production to designated levels. He never gave up on promoting narcotics controls even after the International Opium Convention of 1925. He convinced the League of Nations to intervene in three conferences (1925: Certificate system- the exporter could only sell to a legitimate importer. This was intended to dry up the flow into the market; 1931: Limited production of manufactured drugs, illicit factories then began; 1936: Law enforcement issue, this involved the extradition of drug smugglers and co-operation between countries.) Delevingne continued to promote the prosecution of illicit drug businesses.