City-state

City-states are extremely small sovereign nations, usually only containing a single city after which they are named. They were common in ancient Greece, where they were referred to as poleis (singular polis), and usually had a fair number of rivalries and political alliances, but would ban together during times of foreign threat.

Historic examples include Sparta, Athens and Corinth. Their infighting, along with the infighting of other Greek city-states, was infamous, but so was their ability to ban together to defeat the Persians when that great empire sought to absorb Greece.

Modern examples of city-states include Andorra and San Marino. Another example is Vatican City.