Which two countries share the longest international land border?

The Canada-United States boundary runs from the Atlantic to the Pacific and then continues along the entire perimeter of Alaska, and it is that second stretch that pushes the total past every rival. Much of the western section is famously a straight line, the forty-ninth parallel, agreed by treaty in the nineteenth century when neither side had surveyed the interior in detail and a line of latitude was the simplest thing to draw. Farther east the border follows lakes, rivers and older colonial claims, threading through the Great Lakes and producing a few oddities, such as a corner of Minnesota reachable overland only through Canada. The Alaskan portion came later, after the United States bought the territory from Russia and a panhandle boundary dispute with Britain was arbitrated. The whole thing is unfortified and, for most of its length, marked only by a cleared strip through forest and a line of small monuments.

The Northwest Angle in Minnesota juts above the forty-ninth parallel because early treaty makers misunderstood the geography of the Mississippi's source.