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In war, in the event of the imminent capture of a city, the government/military structure of the nation that controls the city will sometimes declare it an open city, thus announcing that they have abandoned all defensive efforts.
The attacking armies of the opposing military will then be expected not to bomb or otherwise attack the city, but simply to march in. The concept aims at protecting the historic landmarks and civilians who dwell in the city from an unnecessary battle.
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 which is 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.
Several cities were declared open during World War II:
- Brussels in 1940
- Oslo in 1940
- Paris in 1940, from which the French Government fled after it became apparent that they could not defend it
- Belgrade in April 1941
- Penang in December 1941 after the British retreated to Singapore
- Manila in 1942, which the American military abandoned because they could not defend it against the surprise Japanese invasion
- Rome on 14 August 1943, which the defending Italian forces declared unilaterally to be an "open city" following the cessation of Allied bombing. This applied when under attack by the Germans; subsequently Allied forces entered Rome in June 1944 and retreating German forces also declared Florence and Chieti on 24 march 1944 "open cities".
- Athens on 11 October 1944 was declared an open city by the Germans.
Open cities in postwar 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 is rejected by nearly all of Japan's political parties and the ruling government as inherently absurd, since Japan is not in a war, and in the event of war such a decision would be have to be approved by the national government. However, the Social Democratic Party which was in power from 1994 to 1996 supports 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. ISBN 0-446-51258-3, p. 210
- Katz, Robert (2007 [last update]). "An Excerpt from The Battle for Rome: 'Open City'". theboot.it. Retrieved 7 July 2011.
- World War II Chronology 1944[dead link]
- (Japanese) Hiromichi Ikegami et al. "Let's protect Article 9 of the Japanese Constitution by declaring ourselves Defenseless Cities!" Municipality Research Company, 2006. ISBN 4880374504 (無防備地域宣言で憲法9条のまちをつくる)
- (Japanese) Prime Minister of Japan. Is it possible for a city to declare itself an "defenseless"?
- 月刊社会民主, vol. 620, p. 8. 社会民主党全国連合機関紙宣伝局
- 月刊社会民主, vol. 596, p. 2. 社会民主党全国連合機関紙宣伝局