The 100th anniversary of the Easter uprising of 1916 saw the beginnings of a deeper appreciation of the achievements of Sir Roger Casement who was hanged as a traitor in Pentonville prison on 3 August 1916. Over the following century he has never lacked for notoriety, famous as an Irish patriotic martyr, but discussion of his life has frequently focused on his sexuality and revolved around the “Black Diaries” that were covertly used by the British government to blacken Casement’s name and sabotage the campaign against his execution.

The controversy over whether or not the diaries were forged never discredited Casement – in Ireland, if anything, they further sanctified his name as a victim of British machinations – but it did divert attention from his work in exposing the mass murder and enslavement of indigenous peoples in the Congo and Amazon. He detailed how they were not only being mistreated, but actually wiped out by the terror imposed by those seeking to obtain rubber through forced labour.

To understand what Casement was trying to stop, it is best to quote some of the Congolese interviewed by Casement for his report published in 1903 which describes the atrocities being carried out by King Leopold 11 of Belgium and his private army in the Congo. The witnesses are identified only by their initials or are unnamed. R.R. said “I ran away with two old people, but they were caught and killed, and the soldiers made me carry the baskets holding their cut-off hands. They killed my little sister, threw her in a house, and set it on fire.” U.U. gives a similar account of the reign of terror, saying that “as we fled, the soldiers killed ten children, in the water. They killed a lot of adults, cut off their hands, put them in baskets, and took them to the white man, who counted 200 hands…. One day, soldiers struck a child with a gun-butt, cut off its head, and killed my sister and cut off her head, hands and feet because she had on rings.”

A refugee from the rubber producing regions of the Congo interviewed by Casement gave a description of the ghastly mechanism by which people were forced either to collect natural rubber or to die: “We had to go further and further into the forest to find the rubber vines, to go without food, and our women had to give up cultivating the fields and gardens. Then we starved. Wild beasts—leopards—killed some of us when we were working away in the forest, and others got lost or died from exposure and starvation, and we begged the white man to leave us alone, saying that we could get no more rubber, but the white men and their soldiers said: ‘Go! You are only beasts yourselves.’”

Casement stresses that these abuses were exterminating the entire local population as they were slaughtered and their villages and towns burned. As a British consular official, Casement suspected that he was finding out more than the British government would want to know, but he felt he had no choice but to expose what was happening in order prevent it. He wrote later in a letter cited in 16 Lives: Roger Casement by Angus Mitchell that “I burned my boats deliberately, and forced the Foreign Office either to repudiate me, or back my report.”

Casement combined several qualities which made him uniquely qualified to investigate and expose the horrors he saw in the Congo and the Peruvian Amazon. He was a fearless and experienced traveller. Joseph Conrad, who met him in the Congo in 1890, wrote of seeing him “start off into an unspeakable wilderness swinging a crook handled stick ...with two bulldogs”. Conrad met him a few months later, a little leaner and browner, still with his bulldogs and otherwise looking little changed.

Physical courage and moral outrage are not enough to combat the beneficiaries of exploitation and mass murder. A politically sophisticated and cosmopolitan Irish nationalist, Casement saw that his reports alone would not be effective against the powerful commercial and political interests headed by the King of Belgium. He wrote that the rubber profiteers were united “and only systematised effort can get the better of them.” He supported, semi-secretly because of his official position, British and Irish politicians, writers and journalists agitating against Belgian misrule in the Congo. Thanks to his Irish background, Casement said he could understand that exploitation of the weak by the strong and of small nations by big nations has the same basic motivation and mechanics regardless of whether it took place in Ireland, Congo or Peru.

I had heard about Casement and in a hazy way admired him when I was a child, through a brief but dramatic encounter between my grandfather and Casement when he was a prisoner. He had been arrested on a Banna Strand in Kerry after landing from a German submarine three days before the Easter Rising began in Dublin on 24 April. He had been in Germany trying to persuade Irish prisoners to fight against Britain and obtain arms for an insurgency in Ireland.

I knew Casement’s name, though not much else, because when I was seven or eight I was shown a drawing of him hanging on the wall in Myrtle Grove, a Tudor house belonging to my uncle Bernard Arbuthnot just inside the medieval town walls of Youghal in County Cork. I was told that the sketch was by my grandfather Jack Arbuthnot, a Major in the Scots Guards who was also an artist and had drawn Casement in his cell in the Tower of London sometime between his arrest and his execution. The words “Tower” and “execution” caught my attention, but otherwise I was uninterested in the little picture.

My father, Claud Cockburn, was interested in the story, had seen the drawing and had known Jack Arbuthnot, whom he liked. He described him as a genuine kind of High Tory who felt that regulations were all very well for others, but should not “interfere in any way with what seemed good to him at the time.” Though a career army officer, he was also part time journalist – speeding from Fleet Street to Windsor Castle to take up guard duty – as well as a cartoonist and artist. My father believed he was Casement’s full time jailer in the Tower and had sketched the prisoner days before he was hanged, though actually the connection was a little different, and the sketch was made before Casement’s trial.

The connection between Major Arbuthnot and Casement is confirmed by a diary written by Gertrude Bannister, Casement’s cousin. This is now in the National Library of Ireland where it was read by the author and historian Kieran Groeger who kindly passed on to me a copy of the relevant pages in the diary. After Casement’s arrest and imprisonment, Gertrude and her sister had been desperately searching for him, but British officials refused to say where he was held. Eventually, the sisters received a hint that he was in the custody of the Life Guards, which Gertrude thought unlikely, but they went to Whitehall where “at last, we saw a certain Major Arbuthnot who showed courtesy and sympathy.” He told them that they should really apply to the Governor of the Tower and they said they had already done so and had received no reply. He said: "'I will write personally'. He then told us that he had seen Roger. He said he needed clothes and suggested we should send in some.” Casement had been kept for weeks in the same clothes in which he had been arrested in what was presumably an effort to demoralise him.

Gertrude and her sister went to the Tower and waited a long time until Casement was brought in by two soldiers and Major Arbuthnot. Gertrude asked “couldn’t you leave us alone.” By her account, he hesitated and then ordered the two soldiers out of the room and stood outside himself. “The interview was terrible,” she wrote. “Roger thought he was to be shot & that was why we had been brought to say goodbye.” In reality, he was still to be tried and was some months from his execution. He had been told by his interrogators that his family, who were hunting for him all over London, were refusing to see him because of his “treachery”. After some time, Major Arbuthnot returned and told Gertrude that “Roger must go and the soldiers took him away.”

The sketch of Casement which my father and I saw at Myrtle Grove must have been drawn soon after this visit and before he was transferred to Brixton Prison. It has since gone missing, though hopefully it is only misplaced and will be found again one day.