Qin dynasty
Note: King Zhaoxiang of Qin (秦昭襄王) had already been ruling Qin for 51 years when Qin annihilated the Zhou dynasty; however the other six warring states were still independent regimes. Some Chinese historiographers thus used the next year (the 52nd year of King Zhaoxiang of Qin) as the official succession from the Zhou dynasty.
King Zhaoxiang of Qin
This, as expected, did not go down well in Qin. So, the very next year (261 BC), Qin general Wang He (王齕) attacked Shangdang, and the Zhao general, Lian Po (廉頗), led 200,000 men to reinforce and defend the region, starting the biggest and bloodiest conflict between these two most powerful military states. After his vanguard forces suffered numerous setbacks, Lian Po recognized that the Zhao army were underpowered against their Qin enemies in field battles, so he readjusted the strategies and dug in with a 100 "li"-long defensive line near Changping, and decided to wait out and exhaust the Qin supply lines (which were at least three times longer than Zhao's, hence more difficult to maintain). This strategy worked, as the Qin offensives could not effectively penetrate the well-entrenched Zhao positions for over a year. Logistically strained, the Qin army attempted to engage in pitched battles, but Lian Po consistently refused to meet them in open battles. At this point, both sides increased the size of their forces at Changping, with the Zhao numbering 450,000 and the Qin numbering 550,000.