Irish Nationalists attempt to invade Canada, accidentally invade United States instead
The Fenian raids were a series of invasions of Canada by Irish-American civil war veterans based in New York. The ultimate goal was to conquer Upper Canada and exchange it for a free and independent Ireland, because when given the choice between the green and prosperous land next door and the frigid beaver-infested hellhole several thousand miles away the British would cave immediately. In the last of the raids a small party of Fenian veterans struck out for Manitoba to join in with the Franco-Native American rebels of Louis Riel. Things seemed to get off to a good start when they reached the border, looted a trading post and took an abandoned Canadian customs station.
There was just one problem: Remember how we said the customs post was abandoned? Yeah turns out the border had recently been redrawn in that area and the customs/trading outpost they were attacking was now located firmly in the USA. They were all arrested by the American army.