The state of Zhao was the northernmost of the regional states. While the main battles between the particular states were fought in the south of Zhao, King Wuling 趙武靈王 (r. 325-299) tried to enlarge the wealth of his state by expanding to the north that was roamed by the tribes of the Linhu 林胡 and Loufan 樓煩 (see Hu 胡). He established commanderies in the north of the Yellow River bend and protected his new domains by erecting a fortification wall.
After the fall of Chu, the state of Zhao became one of the most powerful states in the Zhou empire, only third after Qin and Qi. General Zhao She 趙奢 even defeated the army of Qin in the battle of Eyu 閼與 (modern Heshun 和順, Shanxi) in 270. Ten years later, the two states competed for the territory of Shangdang 上黨 (modern Qinyang 沁陽, Henan). The army of Zhao was trapped at Changping 長平 (near modern Gaoping 高平. Shanxi), and Zhao surrendered, but the Qin general Bai Qi buried alive more than 40.000 surrendering troops (according to historiography). Yet Zhao was not yet finally defeated - for two more years the army of Qin beleaguered the capital of Zhao, Handan 邯鄲 (modern Handan, Hebei). Lord Xinling 信陵君 of Wei organized a relief army and rescued the state of Zhao, but Zhao was never able to recover from the terrible defeat at Changping.
Over a long period, the dukes and kings of Qin had were supported by competent advisors, first by the legalist politician Shang Yang 商鞅, then counsellor Wei Ran 魏冉 (Marquis Rang 穰侯), and then Fan Ju 范雎, who proposed that Qin should create alliances with distant states against close enemies, and who stressed that the ruler had to replace the princely lineages and other members of the nobility by employees in a state bureaucracy.
Strengthened by the strategies proposed by such advisors, King Zhaoxiang 秦昭襄王 (r. 306-251) was able to occupy large territories of Zhao, Wei and Han and conquered the Sichuan Basin with the states of Shu 蜀 and Ba 巴 which stood outside the political system of the Zhou empire.
Under King Zhuangxiang of Qin 秦莊襄王 (r. 249-247) and his Counsellor-in-chief Lü Buwei 呂不韋, Qin had already advanced far into the territories of the states of the Central Plain and brought the house of Zhou, in an extremely weak position anyway, to an end. The last ruler of the Zhou was the Lord of East Zhou 東周君 (r. 255-249).
In 246, the young prince Ying Zheng 嬴政 succeeded to the throne of Qin, in 237 he took over regency by himself, aided by Counsellor Li Si 李斯, who helped Ying Zheng with his plans to conquer the remaining states. In 230, Qin destroyed Han, two years later Zhao was finally defeated after general Li Mu 李牧 had been assassinated. In 226, Qin advanced to the northeast and conquered Yan, one year later, Qin beleaguered the capital of Wei, Daliang 大梁 (near modern Kaifeng 開封, Henan). The city was destroyed by branching off a canal from the Yellow River. In 223, general Wang Jian 王翦 led 60.000 soldiers to conquer Chu. In the following year Qin finished the conquest of the northeast, destroying Yan and Dai 代, the successor state of Zhao. The last state to tumble was Qi in 221.
Only deep changes within the states had made possible such prolonged phases of military campaigning during the last century before the unification of China by Qin. All rulers of the particular states had an entourage of political advisors that did not only propose actual tactics, but also advocated profound reforms in the administrative structures. Among the most famous persuaders were Su Qin and Zhang Yi 張儀. Some important aristocrats still participated in politics, like Lord Mengchang 孟嘗君 of Qi, Lord Pingyuan 平原君 of Zhao, Lord Xinling 信陵君 of Wei, and Lord Chunshen 春申君 of Chu, but in general the noble families (qing dafu 卿大夫), relatives of the rulers or meritorious families, had lost their political influence and were replaced by state officials (li 吏).
Traditional regional states were replaced by an organization in directly administered commanderies (jun 郡), especially in the newly conquered territories, as can be seen in the northwest of the state of Yan (commanderies Yunzhong 雲中, Jiuyuan 九原) and in the south and east of Qin (commanderies Shu 蜀, Ba 巴, Qianzhong 黔中, Hanzhong 漢中, Nanjun 南郡, Shangjun 上郡, Shangdang 上黨, etc.). Among all states carrying out reform, Qin was the quickest to adapt changes in administration (government, standardization of weights and measures, codification of law), and taxation and military (conscripts from among the peasantry). Furthermore, its location in the region "within" (west of) the Hanguan Pass 函關 (guannei 關內) provided protection from the armies of the states "east of the Pass" (guandong 關東). Last but not least, the state of Qin was governed by a sequence of strong rulers (Duke Xiao 秦孝公, King Zhaoxiang 秦昭襄王 and Ying Zheng 嬴政, the First Emperor of Qin 秦始皇帝), all with competent and ambitious political advisors as Shang Yang 商鞅, Lü Buwei 呂不韋, and Li Si 李斯, and excellent generals as Bai Qi 白起, Wang Jian 王翦 and Meng Tian 蒙恬.