gǔ / dài / shǒu / chéng / shí / jūn / duì / cháng / yòng / jiān / bì / qīng / yě / lái / ruò / lái / fàn / zhī / dí

古代守城时，军队常用坚壁清野来削弱来犯之敌。

In ancient times, armies often used a scorched-earth strategy to weaken invading enemies while defending a city.