Roger David Casement (Irish: Ruair Dith Mac Easmainn; 1 September 1864 3 August 1916), known as Sir Roger Casement, CMG, between 1911 and 1916, was a diplomat and Irish nationalist executed for treason during World War I. He worked for the British Foreign Office as a diplomat, becoming known as a humanitarian activist, and later as a poet and Easter Rising leader. Described as the "father of twentieth-century human rights investigations", he was honoured in 1905 for the Casement Report on the Congo and knighted in 1911 for his important investigations of human rights abuses in the rubber industry in Peru.In Africa as a young man, Casement first worked for commercial interests before joining the British Colonial Service. In 1891 he was appointed as a British consul, a profession he followed for more than 20 years. Influenced by the Boer War and his investigation into colonial atrocities against indigenous peoples, Casement grew to mistrust imperialism. After retiring from consular service in 1913, he became more involved with Irish republicanism and other separatist movements.

During World War I, he made efforts to gain German military aid for the 1916 Easter Rising that sought to gain Irish independence. He was arrested, convicted and executed for high treason. He was stripped of his knighthood and other honours. Before the trial, the British government circulated excerpts said to be from his private journals, known as the Black Diaries, which detailed homosexual activities. Given prevailing views and existing laws on homosexuality, this material undermined support for clemency for Casement. Debates have continued about these diaries: a handwriting comparison study in 2002 concluded that Casement had written the diaries, but this was still contested by some.

A diplomat (from Ancient Greek: δίπλωμα; romanized diploma) is a person appointed by a state or an intergovernmental institution such as the United Nations or the European Union to conduct diplomacy with one or more other states or international organizations.

The main functions of diplomats are: representation and protection of the interests and nationals of the sending state; initiation and facilitation of strategic agreements; treaties and conventions; promotion of information; trade and commerce; technology; and friendly relations. Seasoned diplomats of international repute are used in international organizations (for example, the United Nations, the world's largest diplomatic forum) as well as multinational companies for their experience in management and negotiating skills. Diplomats are members of foreign services and diplomatic corps of various nations of the world.

The sending state is required to get the consent of the receiving state for a person proposed to serve in key diplomatic positions such as an ambassador, also referred to as the head of the mission. The receiving state of the proposed diplomat may accept the diplomat or refuse to accept the diplomat without having to provide reasons for its refusal or acceptance of the person. While the head of the mission or any member of the diplomatic staff is already on duty in the receiving state, the receiving state may still decide at anytime that the person is no longer wanted in the state and is considered persona non grata. When this happens, the sending state may discharge the person.Diplomats are the oldest form of any of the foreign policy institutions of a state, predating by centuries foreign ministers and ministerial offices. They usually have diplomatic immunity, and in their official travels they usually use a diplomatic passport or, for UN officials, a United Nations laissez-passer.