As with almost every geographic “biggest” or “longest”, there are seven different ways to tabulate the results. The longest border between two countries would seem to be that between the U.S. and Canada. In total, it’s about 5,526 miles (8,893 km). However, that total includes two pieces – the one along the Lower 48 and the one along Alaska. The longer piece is about 3,987 miles (6416 km) of that total, so some purists would argue that the longest single border between two neighbors is actually between Russia and Kazakhstan, at 4,254 miles (6,846 km). Well, OK, but here’s the cool thing about the U.S./Canadian border: it’s undefended. We’re friends! Who could be mad at Canada? Despite the bonhomie of this relationship, both sides are fairly keen on knowing just exactly where the line really is. To this end, there is a 20 foot (6m approx) wide swath cut through the endless miles of forest between the two countries, even in the remotest regions.
Most of the border above the contiguous U.S. states is “straight”; at least it appears so on a small scale map. Since it was surveyed in the early 1870s, the technology used was less precise than the lasers and GPS of today. Given the rough country involved, even farther from civilization then than it is now, it is no surprise that the official border doglegged a bit. As the surveyors went, they emplaced 900 markers, and the line connecting these points wanders – in some places it’s more than a few hundred feet off of true. Apparently, in nearly 150 years, it’s not been enough of a problem to worry about.
More strange are the geographic oddities that resulted from attempting to have the 49th parallel define the border all the way from the Lake of the Woods, above Minnesota, westward to the Pacific. Due to ignorance about the actual lay of the land (or the lake, actually), mapmakers made seemingly straightforward instructions for the surveyors that ended up requiring a convoluted border path. Strangely, it cut off a chunk of land along the northern lakeshore that should logically be part of Canada, and included it in the U.S. That outlier is now known as the Northwest Angle. Another oddment is at the far western end of the “straight line” where an unnoticed bit of Canada called Point Roberts hung innocently below the 49th parallel and got sliced off. Again, advantage U.S.A. Canada did do better out on the East Coast where Campbello Island off Maine’s northeastern shore was once in dispute but is now securely Canadian. Still, by land, it’s only accessible over a bridge from Maine.
The border has other complications, like river boundaries that occasionally change their course, but so far there haven’t been any skirmishes over that sort of minor adjustment. In the histories of nations, it’s pretty remarkable that two neighbors have had such a lengthy and amicable line between them for so many years.