At the end of June 1916, when Roger Casement was sentenced to be hanged for treason after enlisting German support for the Easter Rising, the liberal journalist Henry W Nevinson considered shouting out "God save Ireland!" from the back of the courtroom in London. But, in the end, Nevinson remained silent. Might he be imprisoned for saying such a thing? As he was English, did he even have a right to say it? Perhaps the Irish would take against him?
Well-meaning Englishmen often struggle to do the right thing by Ireland. But Nevinson's timidity was far less egregious than that of his other liberal friends, to whom Casement's crusade against slavery and abuse in the rubber plantations of Africa and South America had made him a hero.
Before the trial began, a government minister told London newspaper editors that detectives had discovered among Casement's papers a diary showing that he had been addicted to "unnatural vice". Word soon spread of the "Black Diaries", in which Casement had allegedly noted down in graphic detail his homosexual encounters with men and boys on his travels. Nevinson, however, was unfazed by the revelations and scurried around London campaigning for a reprieve. Nine days before Casement was hanged, he wrote in the Manchester Guardian: "It is common knowledge that insinuations against Casement's private character have been passing from mouth to mouth. These insinuations have no bearing on the charge of which he was convicted, nor have they been established or mentioned in court." But the campaign floundered and Casement was hanged at Pentonville jail on 3 August, his body buried in quicklime in the prison yard, where it lay until his bones were exhumed and returned to Ireland for a state funeral in 1965.
Debate about whether the Black Diaries were forged by the British raged throughout the 20th century. Most experts now believe that the diaries were real. They include Jeffrey Dudgeon, a biographer of Casement, who thinks the documents show that he was "an early exemplar of what is now standard sexual behaviour for most gay men".
For a long time, both the diaries and Casement's involvement in the 1916 uprising obscured that he was an exemplar of another kind - of the international diplomat as humanitarian hero. Casement was the Sergio Vieira de Mello of his day: a kindred spirit of the UN diplomat killed in a hotel bombing in Baghdad in 2003.
He first gained fame in 1904 with his unsparing account of the abuses perpetrated on the Congolese rubber plantations run on behalf of Léopold II, king of Belgium, which he regarded as legalised "piracy". He produced a similar report, published by the House of Commons in 1913, about the slavery imposed on the Putumayo Native Americans in Peru. When his despatches made him famous, everybody in London wanted to meet him.
Born in Dublin to a Catholic mother and a Protestant father, Casement was orphaned in his teens and raised in Ballymena, County Antrim. He followed the career path of many Irish Protestants of his class, administering to the needs of the British empire - he took up a position at a shipping company in Liverpool at the age of 16 and went to Africa to work for trading interests in Congo before falling into a job as the British consul.
In June 1890, Casement was running a trading station at Matadi in the Belgian Congo when Joseph Conrad arrived on the expedition that was to provide the material for his novella Heart of Darkness. All the Europeans Conrad encountered made him feel uncomfortable, but he confided in his diary that he had been impressed by the tall Irishman: "Made the acquaintance of Mr Roger Casement, which I should consider as a great pleasure under any circumstances and now it becomes a positive piece of luck. Thinks, speaks well, most intelligent and very sympathetic."
Thirteen years later, when Casement was in London on the eve of the publication of his Congolese report, Conrad wrote to him recommending that he see his friend R B Cunninghame Graham, writer, adventurer and founder of the Scottish Labour Party. Conrad signed off with: "God bless you and your work in the new year and in the long years to come." But after Casement's arrest, in April 1916, Conrad wrote to the American lawyer John Quinn that he judged the humanitarian crusader to be "a man . . . of no mind at all. I don't mean stupid. I mean that he was all emotion. By emotional force . . . he made his way and sheer emotionalism has undone him." A week after Casement hanged, Ottoline Morrell mentioned him to Conrad. He quickly and coldly closed off the subject.
When, nearly ten years ago, the Peruvian writer Mario Vargas Llosa reviewed King Leopold's Ghost, Adam Hochschild's history of the Belgian Congo, he was struck by the bravery and perseverance of Casement and his collaborator E D Morel in making the abuses in Congo an international scandal.
Both of them, Vargas Llosa wrote, deserved "the honour of a great novel". With the publication in Spanish last month of El sueño del celta ("the cream of the Celt"), he has chosen Casement. But how will Vargas Llosa, who has denounced nationalism as the religion of the demagogue, reconcile the Casement who is a humanitarian crusader with Casement the Irish nationalist?
The key is that Casement's crusade flourished in an era when nationalism and internationalism were intertwined more closely than ever before, as imperial networks carried ideas from one side of the world to the other. When he first arrived in Africa, he held the usual opinions of a man of his class and station. As he explained in a letter to his confidante Alice Stopford Green: "I had accepted imperialism - British rule was to be extended at all costs because it was the best for everyone under the sun."
Casement began to question imperialism during the Boer war. His boyhood fascination with Irish history (the title of Vargas Llosa's novel is that of an epic poem by Casement in which he refers to how Elizabeth I had left Ireland "wet with blood and red with flame") fused with his experience of imperialism in Africa. He wrote to Stopford Green: "Up in those lonely Congo forests where I found Leopold, I found also myself - the incorrigible Irishman . . . I realised then that I was looking at this tragedy with the eyes of another race, of a people once hunted themselves." From then on, Casement turned his job as a British consular official into a platform for attacking abuse, using publicity and journalism in strikingly modern ways.
By 1911 - the year he was knighted - he was urging other humanitarian campaigners "not [to] limit your vision to the African . . . The cause of human freedom is as wide as the world." In retrospect, Casement's views seem at once advanced and rooted in the conventions of his time. He regarded the London Stock Exchange as "a thing to kick and spit upon"; he believed that Islam was socially better suited to Africa than Christianity. But spending time in Brazil with the local people tried his patience and he expressed his frustration in a letter to
a friend in 1910: "I don't want any more Latin Americans for you. Heavens! What loathsome people they are! A mixture of Jew and nigger and God knows what; altogether the nastiest human black pudding the world has yet cooked in her tropical stew pot." And he believed that the right kind of imperialism might destroy the oppression against which he fought.
Casement's Germanophilia was shared by many pre-First World War British intellectuals. Unlike some of his fellow Irish nationalists, he had illusions about Germany that were not just driven by a myopic search for any old allies in a parochial fight against British rule, but the sign of a man ahead of his time and of a mind searching for a new world order.
Maurice Walsh is Alistair Horne Visiting Fellow at St Antony's College, Oxford.
His most recent book is "The News From Ireland: Foreign Correspondents and the Irish Revolution" (I B Tauris, £20)