I’d suspect it would be Canada and Denmark (Greenland),

China?

Russia-Kazahhstan (4254 miles)
source

Canada – Russia….?

Undefended
Argentina and Chile down the Andes Mountains.

@Yellowdog, as an island, Greenland has no land borders. The ocean around an island isn’t actually a border; or if it is, it’s the island separated from everything else and not just from what’s nearest across an expanse of water.

(And how do we know it’s not defended?)

Canada and Denmark (for Greenland) have had an ongoing border dispute for decades. There seems no real resolution over Hans Island. They fight over it every year (in a polite way).

California / Nevada.

Well, technically I would think it would be Canada. For a country, it has the largest coast than any other country. 125,567 miles. Hard to border the ocean.

I found this article , for what it is worth. The U.S-Canada border is undefended in the sense that there is no military presence. U.S. border patrol agents carry weapons, although Canadian border patrol agents do not, according to this article

I was surprised by how long the border between Russia and Kazakhstan is, The border between the U.S. and Canada is longer (world’s longest), because it includes the border between Alaska and Canada, which is fairly long.

Looking at the list of longest borders, they start getting to around 1000 miles toward the end of the list. Since all those borders are defended, the second longest undefended border is pretty small compared to the border between the U.S. and Canada. Are borders between EU countries defended? If not, I would guess the second longest border would be between two EU countries, maybe France and Belgium for a length of a few hundred miles.