Which two nations share the longest undefended border in the world?

The longest undefended border in the world is shared by Canada and the United States, which stretches over 8,891 kilometers (about 5,525 miles). This extensive border was established through a series of treaties and agreements, making it one of the most amicable and peaceful international boundaries.

Both countries have developed strong diplomatic relationships, emphasizing cooperative border management that reflects their shared cultural, economic, and historical ties. The lack of military presence along most of this border is a testament to the trust and collaboration that exist between the two nations.

In contrast, the other options do not match the criteria as closely. For instance, while Chile and Argentina share a lengthy border, it is marked by rugged terrain and has been historically more contentious, leading to military considerations at certain points. Kazakhstan and Russia share a long border, but it also has military presence due to geopolitical tensions. The border between Egypt and Sudan, while significant, is not nearly as long or peaceful as that of Canada and the U.S.