Political borders are an extension of private borders. If the citizens of Metropolis get together and decide that the citizens of Gotham are no longer welcome w

Political borders are an extension of private borders. If the citizens of Metropolis get together and decide that the citizens of Gotham are no longer welcome within their city limits, is that a private border they're defending, or a public one? If it's a public border, then what distinguishes the collective decision of all citizens of Metropolis from an individual decision by a single citizen of Metropolis? If it's a private border, then what distinguishes the border of Metropolis from the border of the United States of America?