I was wondering whether Wikipedia would comment. They don't, but somebody already edited Bhumibol's article to say that "His father was enrolled in the public health program at Harvard University, which is why Bhumibol was the only monarch to be born in the US until the 2025 papal conclave elected Pope Leo XIV."
There are some sources indicating that children of foreign sovereigns would be exempt from automatic citizenship, but Bhumibol's father wasn't the king, just the king's brother.
Éamon de Valera's case is unambiguous.
There are surely other world leaders who spent significant time in the US - Benjamin Netanyahu spent some time in the Philadelphia area as a child, for example. And a little bit of research turns up Naftali Bennett, prime minister of Israel in 2021-22 - he was a US citizen (born in Israel to US citizen parents) until he had to renounce his US citizenship when elected to the Knesset.
Famously Einstein was offered the presidency of Israel (which is a largely ceremonial post), which presumably would have come with Israeli citizenship, but he turned it down.
There are some sources indicating that children of foreign sovereigns would be exempt from automatic citizenship, but Bhumibol's father wasn't the king, just the king's brother.
Éamon de Valera's case is unambiguous.
There are surely other world leaders who spent significant time in the US - Benjamin Netanyahu spent some time in the Philadelphia area as a child, for example. And a little bit of research turns up Naftali Bennett, prime minister of Israel in 2021-22 - he was a US citizen (born in Israel to US citizen parents) until he had to renounce his US citizenship when elected to the Knesset.
Famously Einstein was offered the presidency of Israel (which is a largely ceremonial post), which presumably would have come with Israeli citizenship, but he turned it down.