After the American Civil War, the Fenian Brotherhood began conducting raids in Canada to capture the transportation networks, and attack military targets, and plot to kidnap Canadian officials. Their goal was to force the British Government to grant freedom to the Republic of Ireland in exchange for regaining control of Canada.

https://en.wikipedia.org/wiki/Fenian_raids

Needless to say, even though the US Government didn't do much to stop them, they weren't very successful in those goals, and seem to be mostly forgotten today. The joys of regional genealogical research.