noun
open city declaration
A formal declaration that a city is undefended and will not be militarily contested, used in international law and historical contexts such as World War II.
第二次世界大戦中、パリは無防備都市宣言を行い、戦闘を避けた。
During World War II, Paris made an open city declaration and avoided combat.
Compound of 無防備 (undefended) + 都市 (city) + 宣言 (declaration). The term is a direct translation of the international legal concept of an 'open city'.