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.
Some examples of declarations of an open city include:
Attacking forces do not always respect the declaration of an "open city." This happened, for example, with the German bombardment of Belgrade in April 1941, and the American bombardment of Rome in 1944.

Brussels in 1940, World War II
Paris in 1940, from which the French Government fled after it became apparent that they could not defend it
Manila in 1942, which the American military abandoned because the city had lost strategic value in light of events
Athens on 1944-10-11 was declared an open city by the Germans
Rome on 1943-08-14, which the defending forces declared unilaterally an "open city"; the Allied forces entered Rome in June 1944; the German forces also declared Florence and Chieti "open cities" See also

Laws of war