From
王之涣《
凉州词》(Wang Zhihuan, Tang, early 8th c.):
黄河远上白云间,
一片孤城万仞山。
羌笛何须怨杨柳,
春风不度玉门关 (The Yellow River climbs far into white clouds; a lone fortress, ten thousand ren of mountain. Why should the Qiang flute complain of willows? The spring wind never crosses Yumen Pass).
玉门关 (Yumen Pass / Jade Gate) was the western terminus of the Han Great Wall, gateway to the Western Regions — so remote that the spring wind (= the Emperor's civilizing warmth) never reaches it.