54. Apostrophes with countries and place names

Possessives and Attributives

The distinction is not between states and countries, but between names with a singular form (Paris, Kansas, Cyprus, Barbados) and nouns that take a plural form although they are singular in meaning (United States, Seychelles, Chicago Heights, Philippines). The singular forms make the possessive with the addition of an apostrophe and an s (Paris’s, Kansas’s, Cyprus’s, Barbados’s); for nouns with a plural form, add only the apostrophe for the possessive (United States’, Seychelles’, Philippines’).

www.chicagomanualofstyle.org/qanda/data/faq/topics/PossessivesandAttributives/faq0043.html