The Gendarmerie Corps of Vatican City State (Italian: Corpo della Gendarmeria dello Stato della Città del Vaticano) is the gendarmerie, or police and security force, of Vatican City and the extraterritorial properties of the Holy See. It was founded in 1816 as Corps of Gendarmes by Pope Pius VII, renamed the Central Security Office in 1970, the Security Corps in 1991, and was restored to its original name in 2002.
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The Vatican is limited in size and defensive forces. A nation like the US or an ally could arrest the Pope because the Vatican doesn’t have air superiority etc. (not comparable to mil of nations). I’m not saying it happened; I am just saying it could. Right now the Italian gov is under duress and they would likely stand down re Vatican. We’ll see.
Our people might not have been there arresting him; we might have just paid him a visit to ask him a series of uncomfortable questions and arrested one of his top people or had Italy do it.