What two countries share the worlds longest undefended land border?

Canada and the United States share the world’s longest undefended land border. The boundary stretches roughly 8,891 kilometres (5,525 miles) from the Atlantic Ocean to the Pacific Ocean, with another section separating the United States and Alaska from Canada’s Yukon and British Columbia.

“Undefended” means that the border does not have a continuous line of military fortifications, armed guard posts, or a standing army facing the other country. It does not mean the border is uncontrolled. Both nations operate customs and immigration agencies, designated crossings, patrols, and extensive security cooperation. Travellers must still meet the relevant entry requirements.

The border’s modern shape developed through a series of treaties and surveys, including agreements following the War of 1812 and the Webster–Ashburton Treaty of 1842. The boundary passes through forests, lakes, rivers, and urban areas. One famous example is the Peace Arch between British Columbia and Washington. A related fact often causes confusion: this is the longest international border, but it is not the longest border of any kind on Earth.