also: じょうかのめい
expression, noun
capitulation; surrender; treaty signed under coercion
Idiomatic expression referring to a humiliating surrender or treaty forced upon a defeated party, originally from the image of signing a treaty beneath the walls of a besieged castle.
敵軍に包囲され、ついに城下の盟を結ぶことになった。
Surrounded by the enemy army, they were finally forced to sign a humiliating surrender treaty.
この不平等条約はまさに城下の盟だ。
This unequal treaty is nothing less than a capitulation under duress.
Literally 'alliance beneath the castle', from the historical practice of a besieged lord being forced to sign a humiliating treaty under the walls of his own castle. The exact origin is uncertain, but the phrase is well-established in classical Chinese and Japanese literature.