What is the longest international border in the world?

The Canada–United States border, officially known as the International Boundary, is the longest international border in the world. It is shared between Canada and the United States, the second and fourth largest countries by area, respectively.

The terrestrial boundary (including small portions of maritime boundaries on the Atlantic, Pacific, and Arctic coasts, as well as the Great Lakes) is 8,891 kilometres (5,525 miles) long, of which 2,475 kilometres (1,538 miles) is with Alaska. Eight Canadian provinces and territories (Yukon, British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, and New Brunswick), and thirteen U.S. states (Alaska, Washington, Idaho, Montana, North Dakota, Minnesota, Michigan, Ohio, Pennsylvania, New York, Vermont, New Hampshire, and Maine) are located along the border.

Also, the International Boundary is commonly referred to as the world's longest undefended border, but this is true only in the military sense, as civilian law enforcement is present. It is illegal to pass the border outside border controls. It is required that everyone passing the Canada–United States border must be checked.