Nowadays the US-Canada border is one of the most peaceful international boundaries in the world, but in 1859 the US almost went to war with British North America in what is now Washington State. A war sparked by a pig.
The 1846 Oregon Treaty was poorly worded and it left San Juan Island itself in something of a state of limbo. This island was claimed both by the British Empire and the United States, and for several years American settlers and the Hudson’s Bay Company mutually occupied the island. However, an American frontiersman shot a British pig, and the squabble between neighbors threatened to turn into an international incident. The two powers were ready for armed conflict and, had a British rear admiral not disobeyed orders to engage the Americans, the conflict might very well have turned violent. In the end, it was a bloodless conflict. There were no casualties, excepting, of course, the pig.