Why is Casement tried and executed by the English government in the novel, Valiant Gentlemen?

During the time period in which this novel is set, Ireland is under English rule, which many of the Irish resent. When Casement becomes involved in Irish activism, he tries to enlist Germany to fight against the English, believing they are the only ones strong enough to defeat England. When his acts of treason are discovered, Casement is tried and hanged by the English government.