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 an open city, the opposing military will be expected to peacefully occupy the city rather than destroy it. The concept aims to protect the city's civilians and cultural landmarks from a battle which may be futile.
Attacking forces do not always respect the declaration of an "open city". Defensive forces will use it as a political tactic as well. In some cases, the declaration of a city to be "open" is made by a side on the verge of defeat and surrender; in other cases, those making such a declaration are willing and able to fight on but prefer that the specific city be spared.
According to the Protocol I of the Geneva Conventions, it is forbidden for the attacking party to "attack, by any means whatsoever, non-defended localities".
Kraków was left undefended (except for some small local units) after the Polish 6th Infantry Division marched by the city to the nearby Niepo?omice Forest to set new defensive lines. This led the Mayor of Kraków to declare it an open city on 5 September 1939. The German army entered the city the next day.
Rome was declared open on 14 August 1943 by the Italian government following the cessation of Allied bombing. Subsequently, Allied forces entered Rome in June 1944 and retreating German forces also declared Florence and Chieti on 24 March 1944 "open cities".
Athens was declared an open city by the Germans on 11 October 1944.
Hamburg was declared open on 3 May 1945 by the Germans and was immediately occupied by the British.
Post-World War II Japan
In 1977, a far-left group in Japan--called the "National Open City Declaration Movement Network"--began organizing activists to make cities preemptively declare themselves "defenseless" under the Geneva Convention, so that in the event of war, they would be legally forced to welcome any invasion. This was rejected by nearly all of Japan's political parties and the ruling government as inherently absurd, since Japan was not in a war, and in the event of war such a decision would have to be approved by the national government. However, the Social Democratic Party--which was the junior party of the ruling coalition from 1994 to 1996--supported it.
^Murphy, Paul I. and Arlington, R. Rene. (1983) La Popessa: The Controversial Biography of Sister Pasqualina, the Most Powerful Woman in Vatican History. New York: Warner Books Inc. ISBN0-446-51258-3, p. 210