Germans blackmailed Casement into supporting the Easter Rising

The Irish nationalist Sir Roger Casement, hanged for treason after the Easter Rising of 1916, was blackmailed into taking part by the German army, according to documents released yesterday.

Sir Roger believed the uprising was "wholly futile" but feared the Germans would make him look "worse than a coward" if he refused to join. After agonising about his "hideous" position, Sir Roger landed in Co Kerry off the Aud, a German submarine loaded with arms, three days before the rising. He was quickly arrested, and hanged at Pentonville jail, London, in August 1916.

Documents released by the Public Records Office show that while he was being held supporters in the United States threatened to murder the British ambassador in Washington within 48 hours of Sir Roger being executed. Their letter was signed "Irish-German Americans".

Scotland Yard said an attempt on the ambassador's life might be made, but thought the threats were more likely to be empty and part of a campaign of intimidation by Clan Na Gael, an American-Irish nationalist group.

The full extent of Sir Roger's misgivings about rebellion in Ireland and his feeling that he was trapped by contacts in the German army were made clear in a letter written in March 1916.

He said a Captain Nadolny had threatened that, if he did not agree to German terms for supporting a rising, their offer of 20,000 rifles would be withdrawn and the American-Irish leader, John Dovey, would be told it was Sir Roger's fault the deal had collapsed.

"I was to be held up to my countrymen in Ireland and America for something far worse than a coward ... my position is hideous," he wrote.

Sir Roger's death has provoked decades of controversy over whether his so-called "black" diaries, detailing frequent homosexual encounters, were forged by British intelligence services.

The bundles of documents released yesterday contain a letter from July 1916 showing that the British Government used the diaries to discredit him in Congo – where he had earlier exposed atrocities against Africans – to stop an uprising there if he was executed.