Qin attacked the town of Qinyang of State of Han in 265 BC which made Shangdang (in modern-day southeastern Shanxi province) an exclave land. Qin continued to invade Han in order to occupy Shangdang. Shangdang was strategically placed west of Zhao and its capture would open an invasion route into Zhao. Within four years, the Qin army isolated Shangdang from the rest of Han by capturing the main roads and fortresses across the Taihang Mountains. Shangdang was poised to fall.
Rather than see Qin take Shangdang, Han offered the commandery to Zhao. King Xiaocheng of Zhao (趙孝成王) accepted and dispatched Lian Po and an army to secure the strategic territory from the encroaching Qin. The Zhao army met the Qin army, led by Wang He, in 262 BC at Changping, south of Shangdang. The Zhao suffered several minor defeats during initial confrontation with the Qin forces. Having assessed the enemy, Lian Po decided the only way to defeat the Qin was to wait them out, as Changping was much farther away from Qin territory than Zhao and thus keeping the army supplied would be much more taxing to the Qin.
The Zhao built several fortresses in the summer of 260 BC and then waited for the Qin to go away. The Qin managed to breach the defences once but did not have the strength or equipment to exploit it; nonetheless, the Qin refused to leave. A three-year stalemate ensued.
The Qin sent agents into Zhao and Han to spread rumours/accusations that Lian Po was too cowardly to fight. King Xiaocheng of Zhao was already dissatisfied with Lian Po's strategy. Lian Po was replaced by Zhao Kuo, son of the famous, deceased Zhao general Zhao She. At the same time, the Qin secretly replaced Wang He with the renowned general Bai Qi.
According to legend, the famous General Zhao She on his deathbed had told his wife never to let his son Zhao Kuo command an army. When Zhao Kuo was appointed general, Lady Zhao and minister Lin Xiangru could not persuade King Xiaocheng to rescind the appointment. However, Lady Zhao extracted a promise from the king that the Zhao clan would not be punished if Zhao Kuo failed.
Zhao Kuo assumed command in July 260 BC of an army reinforced to approximately 400,000 men. Zhao Kuo took part of his army and attacked the Qin camp. Bai Qi responded with a Cannae-like manoeuvre. Part of the Qin army withdrew toward the Qin fortress, drawing Zhao Kuo after them. A body of 25,000 cavalry, and 5,000 light cavalry with bows and crossbows, remained behind to spring the trap.
When the Zhao attack reached the Qin fortress, the Qin cavalry ambushed the Zhao Kuo's rear while the Qin light cavalry surrounded the Zhao fortress. With the enemy trapped, Bai Qi launched a counterattack. The Zhao army was split in two and its supply lines cut. Zhao Kuo was unable to continue his attack or return to the Zhao fortress; his army dug in on a hill and awaited relief.
Since 295 BC, Zhao foreign policy had been dominated by opportunism and had frequently shifted between hezong (合縱) (anti-Qin alliances) and lianheng (連橫) (pro-Qin alliances). Thus, as the battle in Changping unfolded, Zhao was unable to secure support from either the State of Chu or the State of Qi. King Zhaoxiang of Qin used this opportunity to mobilize additional forces against Zhao from Henei, in modern-day Henan province; he bestowed one grade of noble rank on the population and ordered all men over the age of 15 to Changping to bolster the encirclement.
Zhao Kuo's hill fortification was besieged for 46 days. In September, having run out of food and water, his desperate army made several unsuccessful attempts to break out. The general was killed by Qin archers while leading his best troops. The Zhao army finally surrendered.
Bai Qi ordered the captured soldiers to be executed, presumably by being buried alive; the local population was hostile to Qin rule, and he was afraid the captured soldiers would revolt. 240 of the youngest soldiers were released to spread terror in Zhao. Sima Qian claimed over 450,000 Zhao soldiers were killed during and after the battle. Emperor Xuanzong of Tang (685-762) later built a temple over a collection of some of the human remains, and bones continue to be discovered on the site. [dead link]
Prior to the campaign, Zhao had been one of the most powerful of the Warring States. The campaign addressed the immediate Qin threat, as three years of war financially and domestically exhausted both states. However, Zhao never recovered from the defeat, which allowed Qin to gain military dominance over the other states. In 221 BC, Qin would use this dominance to unify China.
The 2004 Chinese television series Changping of the War is based on the battle. The battle also features as background to the events taking place in the manga Kingdom with the events having a direct and indirect impact on the character development of several major characters on many sides, most notably Mangoku of Zhao, who led an army of other survivors and families of those killed in bloody and gory revenge upon Qin citizens.