in international law, a city that has been declared undefended by one of the warring states and therefore cannot be a military theater.
The legal status of an open city is based on the statute entitled Respecting the Laws and Customs of War on Land, which was adopted at the Hague Conference of 1907. Article 25 of the statute forbids “the attack or bombardment of towns, villages, habitations, or buildings which are not defended.” During World War II (1939–45), Paris and Rome, for example, were declared open cities.