On 3 August 1916, Roger Casement was hanged at Pentonville Prison in London for high treason. Five years earlier, he had been knighted for his work as a British consular official.

Casement had spent more than twenty years in British government service. In 1903, he investigated conditions in the Congo Free State, which was controlled personally by King Leopold II of Belgium. His report documented killings, mutilation, hostage-taking and forced labour connected with rubber production.

He later investigated the treatment of Indigenous workers in the Putumayo region of the Amazon. A British-registered company was involved in a system that used violence and debt to force people to collect rubber. Casement’s evidence increased pressure for action against those responsible.

The British government recognised his work, appointing him a Companion of the Order of St Michael and St George and later awarding him a knighthood.

His investigations also strengthened his opposition to imperial rule. After retiring from the consular service, he became more active in Irish nationalism and supported the Irish Volunteers.

When Britain entered the First World War against Germany, Casement travelled to Germany to seek support for Irish independence. He tried to recruit an Irish Brigade from prisoners of war who had served in the British Army. Few agreed to join.

Casement also negotiated for German weapons to be sent to Ireland. By April 1916, however, he believed the planned Easter Rising had little prospect of success without greater military assistance.

He returned to Ireland in a German submarine, intending to warn nationalist leaders against proceeding. He landed near Banna Strand in County Kerry on 21 April and was arrested soon afterwards. The ship carrying German rifles was intercepted and scuttled by its crew.

Casement was taken to London and tried under the Treason Act 1351. The prosecution argued that he had adhered to the king’s enemies by seeking German assistance and recruiting among British prisoners.

The defence did not deny his activities in Germany. It argued that the medieval statute did not cover acts committed outside the king’s realm. The judges rejected that interpretation. Casement was convicted on 29 June and sentenced to death. His appeal failed.

Writers, politicians and public figures appealed for clemency. During the campaign, private diaries attributed to Casement were shown or described to some potential supporters. Their accounts of sexual encounters, at a time when sex between men was criminalised, damaged his prospects of receiving a reprieve. The diaries were not evidence in the treason trial.

Casement was executed at nine o’clock on the morning of 3 August. His body was buried within Pentonville Prison. In 1965, the British government released his remains for reburial in Ireland. They were placed in Glasnevin Cemetery after a state funeral.