Which two countries share the longest international border?

The longest international border in the world is shared between Canada and the United States, extending approximately 8,891 kilometers (5,525 miles). This border includes a combination of land and water boundaries and is notable not only for its length but also for the peaceful relationship between the two countries, making it one of the most secure and well-managed borders globally.

The border facilitates extensive trade and movement, enhancing economic ties and cultural exchange. Unlike other international borders that may have more restrictive regulations or historic conflict, the Canada-United States border reflects a unique partnership and collaboration in managing border security, immigration, and commerce.

The other options represent borders that, while significant, do not surpass the length of the border between Canada and the United States. For instance, the border between Mexico and the United States is considerably shorter, as are those between India and Pakistan, and Brazil and Argentina.