Battle of Changping

In 262 BC, the Qin army, led by Wang He, invaded Shangdang, and Feng Ting had to evacuate the commandery. The Zhao army also mobilized, and after arriving at the front line, Lian Po set up three defensive lines pivoted south of Changping Pass (at the border between present-day Gaoping and Zhangzi County).

In April, the Zhao army first encountered the Qin army west of Gaoping Pass (at the border between present-day Gaoping and Qinshui County, Shanxi) on Lian Po's first defensive line, and suffered several defeats during initial confrontations with the Qin vanguards. The Qin army killed the Zhao commander in Gaoping and quickly capitalized on the momentum of these successes, capturing Gaoping Pass as well as three other Zhao strongholds nearby, and the first defensive line collapsed. Having assessed the enemy's strength, Lian Po concluded the only way to counter the Qin offensive was to avoid field battles and hold the second defensive line along the Dan River (丹水, the largest tributary of Qin River, which traverses the present-day Gaoping city). He started focusing on further reinforcing positions on the east bank of Dan River valley using fortified ramparts built along the mountain foothills, hoping to exhaust the Qin army, as Changping was much farther away from Qin territory than Zhao and keeping the army supplied would be much more taxing to the Qin due to the more rugged and winding mountain routes in the west.

The Qin army did attempt crossing the Dan River and even once managed to breach Lian Po's second defensive line, but they did not have enough strength to exploit it and were beaten back. By July, the two sides were forced into a bitter stalemate across the river, which lasted the next two years. Both sides massively reinforced their positions, with total combatants numbering around a million in early 260 BC.

While Lian Po's strategy of holding and wearing out the Qin army was working, problems started to arise back home. The Qin side were frustrated and desperate to break the stalemate, so they sent spies into Zhao and Han to spread rumors that Lian Po was too senile and cowardly to fight. King Xiaocheng of Zhao was already dissatisfied with Po's strategy of dragging out the war for so long, which was also very logistically taxing for Zhao, so he decided to replace Lian Po with Zhao Kuo, the overconfident yet untested son of the famous late general Zhao She.

According to Shiji, the young Zhao Kuo excelled in reciting military philosophies so much that his father often got flabbergasted in debates. However, on his deathbed Zhao She had told his wife to never let his son command an army, because Kuo regarded wars as easy games and treated risks with hubris rather than caution, despite having never experienced any actual battles. When Zhao Kuo was appointed general, Lady Zhao and minister Lin Xiangru tried to persuade King Xiaocheng to rescind that appointment, but their appeal failed. Lady Zhao however did manage to extract a promise from the king that the Zhao clan would not be punished if Zhao Kuo loses the war.

Upon hearing that the Zhao king fell for the rumor, the Qin secretly replaced Wang He with the renowned general Bai Qi, who was known for being brutally efficient in annihilation battles. The appointment of Bai Qi was highly classified in order to not alarm the Zhao army, and anyone who leaked the news would be punished by death.

In July 260 BC, Zhao Kuo arrived at Changping and took over command of the Zhao army. He discarded all previous defensive strategies made by his predecessor, and instead decided to take his main force north to quickly cross the Dan River and attack the left flank of the Qin army stationed along the river's west bank, hoping a decisive offense would be enough to break the Qin army. In doing so, he left most of the provisions stored at the old main camp further south along the river, which was now relatively undermanned and could not secure the elongated supply line stretched along in front of the Qin army across the river.

Unknown to Zhao Kuo, Bai Qi had anticipated his plan and responded with a maneuver that resembled the later Battle of Cannae. Bai Qi had deliberately reduced the strength of his left flank stationed along the river, and had a line of hill fortifications further west of the river reinforced instead. When Zhao Kuo crossed the river and attacked, the Qin left flank quickly abandoned their riverside positions and withdrew back west toward the hills, drawing Zhao Kuo to chase after them. However, a Qin detachment of 25,000 men had then traversed north through the Taiyue Mountains to perform a wide left encirclement behind Zhao's third defensive line stapled around the Changping Pass, cutting off Zhao supply lines from the north. At the same time, another detachment of 5,000 light cavalry with bows and crossbows advanced on the right and crossed the Dan River to cut off communications between Zhao Kuo's new camp and their southern main depot, splitting the Zhao army into two, and Bai Qi's main force immediately followed in to secure and choke off the Dan River valley exits. These detachments eventually completed a triangular encirclement that trapped Zhao Kuo's forces around a small mountain range.

Zhao Kuo soon found his attack against the enemy left flank halted by the Qin hill fortifications, unable to make any progress. He then realized that his rear were being ambushed by the Qin cavalry and provisions were quickly running low, and was forced to abandon his offensives and pull back across the river. The Qin army then counterattacked and pursued, inflicting heavy casualties upon Zhao Kuo's retreating army. After being unable to reach the friendly forces in the south, the Zhao Kuo's army dug in on a hill and had to await relief.

However, 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), depleting its diplomatic goodwills with other states. Therefore, as the battles in Changping unfolded, Zhao was unable to secure any reinforcement 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), by bestowing one grade of noble rank on the population as merits and ordered a nationwide mobilization conscripting every able-bodied man over the age of 15, with the king himself personally overseeing the reserves to the Changping frontline in order to further bolster the encirclement.

With the enemy firmly trapped, Bai Qi started repeatedly launching attacks to further wear out the Zhao army and deny them any chance of escape. Zhao Kuo's improvised hill positions were besieged non-stop for 46 days, and by September, his army's struggle for survival grew more desperate, with the starving Zhao soldiers slaughtering all the horses and allegedly even murdering and feeding on each other. The fighting was also so fierce that half of the Qin soldiers were killed in combat, but the exhausted and demoralized Zhao army was ultimately unsuccessful in breaking out. Zhao Kuo was eventually killed by Qin archers when leading his best troops in a final attempt to breach the encirclement. With their commander dead, the remaining Zhao army gave up and surrendered.