When the US and Canada went to war against each other. Copied from the Wikipedia page "Pig war".
The Pig War was a confrontation in 1859 between the United States and the United Kingdom over the British–U.S. border in the San Juan Islands, between Vancouver Island (present-day Canada) and the State of Washington. The Pig War, so called because it was triggered by the shooting of a pig, is also called the Pig Episode, the Pig and Potato War, the San Juan Boundary Dispute, and the Northwestern Boundary Dispute. Despite being referred to as a "war" there were no casualties on either side, aside from the pig.
https://en.wikipedia.org/wiki/Pig_War_(1859)