What is the Canada-United States border?

The Canada-United States border is the longest international border in the world at 8,891 kilometres including all maritime sections, and 6,416 kilometres on the continental land border excluding maritime sections. The border runs from the Bay of Fundy in the east across the Great Lakes and the prairies to the Pacific Ocean in the west, then from the Pacific north through Alaska to the Beaufort Sea. The land border is unfortified except by markers and is the longest demilitarised border in the world.

The border was established in stages by treaty over more than a century. The 1783 Treaty of Paris (ending the American Revolutionary War) drew the original eastern border. The 1818 Convention between the United Kingdom and the United States set the border at the 49th parallel from Lake of the Woods west to the Rocky Mountains. The 1846 Oregon Treaty extended the 49th parallel border west to the Pacific (modifying the proposed Columbia River boundary). The 1903 Alaska Boundary Tribunal settled the Alaska Panhandle boundary. The 1908 International Boundary Commission was established to maintain the border, operating jointly with the United States ever since.

The border crosses many distinct geographic features. The St. Croix River and the highlands of Maine separate New Brunswick from Maine. Lake Champlain and the upper Connecticut River separate Quebec from Vermont and New Hampshire. The St. Lawrence River, Lake Ontario, the Niagara River, Lake Erie, and the Detroit River separate Ontario from New York, Pennsylvania, Ohio, and Michigan. Lake Huron, the St. Marys River, and Lake Superior separate Ontario from Michigan and Minnesota. The 49th parallel runs from the Lake of the Woods west to the Pacific. Alaska's southern border with British Columbia follows the watershed and a portion of the Portland Canal.

The border has 119 official land crossings and many smaller water crossings. Major crossings include the Detroit-Windsor Tunnel and the Ambassador Bridge between Detroit and Windsor (the busiest crossing, with about 25 per cent of Canada-US trade by value), the Peace Bridge at Niagara Falls, the Lewiston-Queenston Bridge, the Champlain Bridge between Vermont and Quebec, the Pacific Highway crossing at Surrey-Blaine, and the Coutts-Sweetgrass crossing between Alberta and Montana. The Gordie Howe International Bridge between Detroit and Windsor (under construction since 2018, scheduled for completion in 2025) will be the longest cable-stayed bridge in North America. The 1908 International Boundary Commission maintains a 6-metre-wide cleared 'vista' along the entire continental border. Border-related issues including refugee policy (the Safe Third Country Agreement of 2002, expanded in 2023), trade, and security continue to shape Canada-US relations.