From 蒋士铨's Qing-dynasty drama 《冬青树·提纲》: 半壁江山,比五季朝廷尤小 — 'half the territory, smaller still than the courts of the Five Dynasties'. Originally mourned the Southern Song's reduced realm after losing the north.
usage
Historically: territory lost to invasion. Figuratively today: a dominant share of a market, an industry, or any 'domain'. Modern usage is often metaphorical rather than about actual land.