The desire to liberate Ireland from British rule was intense among the Irish in America, many of whom had fled Ireland during the Great Famine. The collapse of the Young Ireland Movement of the 1840s only intensified this feeling.
The Fenians took their name from ancient Irish warriors, and members were generally sworn to secrecy. Fenianism's international aspect, and its clandestine membership, made it seem especially ominous to the British.
There were rumors that the US Army's Irish Brigade in the American Civil War was actually a training ground for a Fenian army that would invade Ireland and drive out the British. And while that grandiose plan never came to fruition, Fenians did attempt an uprising in Ireland in early March 1867.
The Fenian Rising, as it was known, was badly organized and disrupted by informers, and was generally a disaster. However, the treatment of Fenian prisoners by the British authorities galvanized public opinion in Britain and Ireland, and did eventually lead to a strong nationalist movement in Ireland.
