The war began around 961 BC, in the sixteenth year of King Zhao's reign, when either one of the Chu vassal states attacked Zhou territory or Zhao launched a preemptive strike. As hostilities broke out, the Zhou official Bo Maofu was ordered to patrol the Han River, to prevent any enemy groups from flanking the Zhou defenses or bypass them completely to invade the vulnerable western holdings of the Zhou, while the southern Zhou vassal states of Zeng, E, Fang, and Deng were inspected and enlisted for the war effort. When the Zhou royal forces arrived from Chengzhou in the middle Yangtze region, they set up their camp at Zeng. From there, the combined troops of the royal armies and the vassal states conquered the area north of the Yangtze, and then crossed the Han River, where King Zhao encountered a rhinoceros, which was interpreted as an auspicious sign. The Zhou army proceeded to subdue the 26 Chu vassal states of the Han River valley, and then attacked and captured the fortified Chu capital near the Jing Mountains. In doing so, the Zhou captured much loot, especially precious metals, supporting the theory that one major reason for Zhou's southern expansion was the quest for ores or plunder. The king was unable to or chose not to destroy or occupy Chu, however, so that Chu was able to rebuild its strength. Nevertheless, the Zhou forces managed to take full control of the area east of the Han River and north of the Yangtze. There, they constructed the stronghold of Lutaishan as political and military base.