In war, an open city is a settlement which has announced it has abandoned all defensive efforts, generally in the event of the imminent capture of the city to avoid destruction. Once a city has declared itself open, the opposing military will be expected under international law to peacefully occupy the city rather than destroy it.
The concept of an open city emerged during World War I, where cities were declared demilitarized in an attempt to avoid destruction. Protocol I of the Geneva Conventions forbids the attacking party to "attack, by any means whatsoever, non-defended localities". The intent is to protect the city's civilians and cultural landmarks from a battle which may be futile.