Which two countries share the longest international border?

Canada and the United States, about 8,890 kilometres including Alaska - and long stretches of it are a straight line along the 49th parallel, drawn by treaty in 1818 across territory neither party had surveyed.

Straight lines on maps become strange on the ground. The border cuts through the middle of a library in Vermont and Quebec, where the reading room is in one country and the books in another and a line of tape runs across the floor. Point Roberts is a piece of the United States reachable by land only through Canada, because the parallel clipped the end of a peninsula. And a twenty-foot-wide cleared strip, the Slash, is maintained through forest along its whole length so the line stays visible.

It is often described as the world's longest undefended border, and that has become dated rather than false. It was genuinely lightly policed for most of the twentieth century, when crossing needed little more than a driving licence; passports became compulsory in 2009. The two countries also once had war plans against each other well into the 1930s - the American one, War Plan Red, involved invading Canada, and Canada's Defence Scheme No. 1 involved pre-emptively occupying Seattle.