Continent is the top of the tree

Seven codes cover the entire planet.

GET /continent/na
{ "continent": "NA", "name": "North America", "population": 609340107, "area": 24709000 }

Same code, one level down, and the whole continent's country list comes back.

GET /continent/na/countries
{
  "continent": "NA",
  "countries": [
    { "country": "AI", "name": "Anguilla" },
    { "country": "AG", "name": "Antigua and Barbuda" },
    { "country": "CA", "name": "Canada" },
    { "country": "CR", "name": "Costa Rica" }
  ]
}

Seven continent codes, every country under each one, no second lookup and no filtering a global country list yourself. Pass in NA and you get North America. Pass in OC and you get Oceania. Same shape either way.

The top of the tree

/continent filters the same country table. Every row already has a full record at /country/{code}, capital, currency, borders and all. One table, not two to keep in sync. The continent response is the entry point when you want the whole region at once.

Try it

Call /continent/{code}/countries for NA, EU, AS, AF, SA, OC, or AN. That's the whole list. There isn't an eighth.