You may have heard the stories in your family. "Great-uncle Patrick was a Fenian." It may have been whispered in secrecy or bellowed with pride. Who were the Fenians? What was their agenda? Why did Canada fear them? Following the unsuccessful 1848 uprising of the Young Irelanders, many members fled Ireland for France, then the US. Two former Young Irelanders, John O'Mahony and Michael Doheny, landed in New York City. They discovered a strong sense of nationalism among the Irish-Americans, and used it as a springboard to form the Fenian Brotherhood. The Fenian Brotherhood is known by several names: Fenians, the Fenian Society, Irish Republican Brotherhood and the Irish-American Brotherhood. There is a discrepancy surrounding their exact date of formation, but general consensus points to 1858. Their name comes from the Fianna - an ancient band of warriors who roamed Ireland in service to the high kings. The Fenians were a non-secular group who's only membership criteria was an unwavering belief in Ireland's total separation from Great Britain, "sooner or never." They were a political and military entity. Their oath was: "I , ___ , solemnly pledge my sacred word of honor as a truthful and honest man that I will labor with earnest zeal for the liberation of Ireland from the yoke of England and for the establishment of a free and independent government on Irish soil; that I will implicitly obey the commands of my superior officers in the Fenian Brotherhood, that I will faithfully discharge the duties of my membership as laid down in the constitution and bylaws thereof; that I will do my utmost to promote feelings of love, harmony, and kindly forbearance among all Irishmen; and that I will foster, defend and propagate the afore-said Fenian Brotherhood to the utmost of my power." O'Mahony practiced as a lawyer in NYC, while Doheny became a staff-writer at the "Irish-American" newspaper. They toured the country, recruiting members and raising funds. Doheny wrote articles detailing Irish oppression at the hands of the English. Membership growth, however, was stifled by the outbreak of the US Civil War (1861-1865). Many Irish fought bravely on both sides of the conflict, while Fenian members gained invaluable military experience. One of the Fenians most notable members was Captain Michael Corchoran, of the famed 'Fighting 69th.' The Fenians capitalized on their newfound military expertise by developing a plan for an armed invasion of Canada in 1866. Their mission -- capture Canada and hold as ransom in exchange for England's granting full freedom to Ireland. Author Seamus McManus states in "The Story of the Irish Race" that the US government was not only fully aware of these plans, but even encouraged the invasion. This isn't too surprising considering the vast amount of money used for the campaign and the sheer numbers of ex-Union soldiers who took part in it. NEXT: The Invasion of Canada