I think it provides more insight if you also rely on the wider historic context going back centuries - so, for the papacy there's the long historic era of the Papal States. These begin to disappear in the 19th century with the appearance of the united kingdom of Italy. My point there being, the Pope had a very active secular role in governance of some actual geographic territory. He was a ruling monarch as well as a reigning pope. Even after the disappearance of the Papal States, various popes continued with some of the monarchical role as far as granting nobiliary titles. These papal titles were sometimes "ad personam" but also frequently hereditary. And since you are interested in American connection to the Papacy, there's a lesser one in the past with the Pope granting nobiliary titles to Americans. Did you know that the mother of the late American president, Rose Kennedy, was created a papal countess? A small footnote in the history of the monarchs of the Vatican but interesting. I don't know if it was hereditary or "ad personam", but if the former then presumably her granddaughter Caroline may be a papal countess to this day. Who knows?

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I know that several American-born persons have become royals (e.g. Grace Kelly, Lisa Halaby) or heads-of-state (e.g. Eamonn de Valera) in foreign countries. But the said royals have been consorts, not sovereigns; and the said sovereigns were not royal.
Would Pope Leo XIV, then, qualify as the first American to become a foreign royal sovereign -- and if you will, a *king*? After all, the papacy is a monarchy: it's just that the pope is obviously an elected monarch. Indeed, the Chinese word for pope is "Church Emperor".