A brief pedantic moment: Doesn’t the expression “World’s Longest Undefended Border” imply that there are longer, defended borders?
There aren’t. Lots of countries have longer total land boundaries than Canada or the U.S., but the next closest shared border length, that of the Russia - Kazakhstan border, is over 2,000 km shorter.
Knowledge is power …
The land boundary lengths from the 2002 World Factbook break down like this:
| Border | Length (km) |
|---|---|
| U.S. - Canada | 8,893 |
| Russia - Kazakhstan | 6,846 |
| Argentina - Chile | 5,150 |
| China - Mongolia | 4,677 |
| India - Bangladesh | 4,053 |
| China - Russia | 3,605 |
| Russia - Mongolia | 3,485 |
| Brazil - Bolivia | 3,400 |
| China - India | 3,380 |
| United States - Mexico | 3,141 |
| India - Pakistan | 2,912 |