MP is two places
MP is Madhya Pradesh in India and the Northern Mariana Islands in the US, same code, same endpoint, two different records.
Madhya Pradesh has never heard of the Northern Mariana Islands. They just happen to share a code.
GET /state/MP?country=IN
{ "country": "IN", "country_name": "India", "state": "MP", "name": "Madhya Pradesh", "type": "state" }
GET /state/MP?country=US
{ "country": "US", "country_name": "United States", "state": "MP", "name": "Northern Mariana Islands", "type": "outlying area" }
Same endpoint, same code, two different records, one in South Asia, one in the middle of the Pacific. ?country= is the only thing standing between them.
Every lookup needs it, not just the ambiguous ones
State and province codes repeat across countries, ISO 3166-2 reuses letters freely. /state/{code} requires ?country= on every call, not just the ambiguous ones, there is no way to know in advance which codes will collide, so it just assumes all of them might. /country/{code} skips the question entirely. One code, one country, no chaperone required.
Try it
Call MP under India, then under the US. Yeah, it's a coincidence. Codes run out eventually.