The Fenian raids were a series of incursions carried out by the Fenian Brotherhood, an Irish republican organization based in the United States, on military fortifications, customs posts and other targets in Canada (then part of British North America) in 1866, and again from 1870 to 1871. A number of separate incursions by the Fenian Brotherhood into Canada were undertaken to bring pressure on the British government to withdraw from Ireland, although none of these raids achieved their aims.
In Canada, the incursions divided its burgeoning Irish-Canadian population, many of whom were torn between loyalty to their new home and sympathy for the aims of the Fenians. Protestant Irish immigrants were generally loyal to the British and fought with the pro-Union Orange Order against the Fenians.
While authorities in the United States arrested the men and confiscated the arms of the Fenian Brotherhood, there was speculation that some in the U.S. government had ignored the preparations undertaken by the Fenians because of anger over British actions that were construed by some as assistance to the Confederate States of America during the American Civil War. The Fenian raids were one of the factors that led to Canadian Confederation, as the provinces united to face the threat of the Fenian invasions.
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https://en.wikipedia.org/wiki/Fenian_raids