Towns or cities that are undefended or open (the terms are used more or less synonymously) have been the subject of regulation, and confusion, in international law and military practice since at least the late nineteenth century.

The laws of war specify that undefended places should not be attacked. The 1907 Hague Regulations on Land Warfare, still formally in force, state in Article 25: "The attack or bombardment, by whatever means, of towns, villages, dwellings or buildings which are undefended is prohibited." Similar language in the 1907 Hague Convention deals with naval bombardment.

The term undefended can be interpreted to encompass all places that are not fortified and do not have an active military presence. In this logic, even a town that is in the interior of a country at war and which has extensive armaments factories or military communications systems could count as undefended. However, the view that such places should be immune from attack has generally been rejected by military planners, especially by airmen, and should not be taken as indicative of prevailing law.

In practice, the words undefended and open have been defined restrictively, to mean simply places declared to be open for entry and occupation by an adverse party without resistance. In this view, the core meaning of the rule prohibiting attacks on undefended towns is simply that a town in a war zone that has declared itself ready to accept the entry of the adversary's army may not be bombed or subjected to artillery attack. This is widely agreed to be the most persuasive interpretation of the practical meaning and original intention of Article 25 of the 1907 Regulations. This restrictive view has been confirmed in the 1977 Additional Protocol I to the Geneva Conventions, Article 59, which says that a belligerent may declare as a nondefended locality "any inhabited place near or in a zone where armed forces are in contact which is open for occupation by an adverse Party."

There have been many cases of towns being declared "open" in this sense. When French forces abandoned Paris in June 1940, the Germans were notified that the city was open for their entry. In June 1944 the German command in Italy asked the Allies to "confirm" the status of Rome as an open city, and then made a unilateral declaration to that effect, followed by surrender of the city to the Allies. Sometimes the concept of open cities has been used differently, to refer to the idea that certain towns should be spared bombardment, even if they are not open to occupation—for example, if they are far from the front line.

Most destructive attacks in modern war have been against cities that were not undefended in the sense of open to occupation by the adversary. This has strengthened the pressure to develop other bases for protecting cities, and their inhabitants, from the ravages of modern war.

The term open cities has been used by the UN High Commissioner for Refugees (UNHCR) with a completely different meaning: it refers to an initiative in Bosnia-Herzegovina following the 1995 Dayton peace agreement to reward local authorities who declare their Opstina, or district, open and are committed to the return of minorities to their prewar homes.

(See legitimate military targets; safety zones.)