Countries on Earth in the 20th century
A country (or nation-state) was a self-governing entity that was generally smaller than a planetary government, and made up of smaller subnational entities (states or provinces), as was found in many instances on Earth.
The United Federation of Planets required that planets have a unified government in order to be considered for Federation membership. However, planetary citizens often felt an attachment to their native region, which may have previously been a nation-state. (TNG: "Attached")
The novelization of Star Trek states that the Riverside Shipyard is sometimes visited by "suits" - a common slang term for government officials - from Washington, Moscow, and Beijing. As these cities are, respectively, the capitals of the United States, Russia, and China - three of Earth's largest and most powerful nations - and have comparatively little industry (especially Washington, having been built for the sole purpose of being the capital city), this would suggest that, even with a fully united Human race, most Earth nation-states still existed in some form in the 23rd century.
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