A country lookup is often the start of a larger question. If you're showing information about the United States and want to let someone explore neighboring countries, you shouldn't have to maintain another list just to connect those pages.
GET /country/US includes these fields on every plan:
{
"country": "US",
"name": "United States",
"borders": ["CA", "MX"]
}
Each entry in borders is a country code you can use directly: /country/CA for Canada, /country/MX for Mexico. Those ordinary lookups return the same country response with their own neighbor lists.
The field describes land borders. It doesn't include a country just because it's a short flight away or across the water. Japan, for example, returns borders: null.
For a UI, handle that null as "no neighbor list" and skip the links. Don't turn it into an is_island calculation. That would make the field answer a geographical question it wasn't designed to answer.
The country endpoint accepts both two-letter and three-letter country codes. borders consistently gives you the two-letter form.