At that time, the southern part of China was divided into two areas controlled by two other warlords, Liu Bei and Sun Quan.

Later that year, on 11 December, Cao Pi forced Emperor Xian to abdicate in his favour and took over the throne, establishing the state of Wei.

Sun Quan was nominally a vassal king under Wei, but he declared independence in 222 and eventually proclaimed himself "Emperor of Wu" in 229.

[citation needed] Between 228 and 234, Zhuge Liang, the Shu chancellor and regent, led a series of five military campaigns to attack Wei's western borders (within present-day Gansu and Shaanxi), with the aim of conquering Chang'an, a strategic city which lay on the road to the Wei capital, Luoyang.

[citation needed] On its southern and eastern borders, Wei engaged Wu in a series of armed conflicts throughout the 220s and 230s, including the battles of Dongkou (222–223), Jiangling (223) and Shiting (228).

[citation needed] After Guanqiu Jian failed to subjugate the Gongsun clan of the Liaodong Commandery,[9] it was Sima Yi who, in June 238, as the Grand Commandant (太尉), launched an invasion with 40,000 troops at the behest of Emperor Cao Rui against Liaodong,[10] which at this point had been firmly rooted under Gongsun control for 4 decades.

After a three-month long siege, involving some assistance from the Goguryeo Kingdom, Sima Yi managed to capture the capital city of Xiangping, resulting in the conquest of the commandery by late September of the same year.

Although the king evaded capture and eventually settled in a new capital, Goguryeo was reduced to such insignificance that for half a century there was no mention of the state in Chinese historical texts.

This event marked the collapse of imperial authority in Wei, as Cao Fang's role had been reduced to that of a puppet ruler while Sima Yi wielded state power firmly in his hands.

[citation needed] Cao Pi felt that the Han dynasty collapsed because the Governors (州牧) of the various provinces wielded too much power and fell outside the control of the central government.

When China was divided in the Period of Disunion, south and north were economically and socially dominated by an aristocratic hereditary class enshrined in law, who were exempt from conscript labor, special kinds of taxes, had legal immunities and other privileges.

This situation was created by Cao Wei with rigid social stratification backed by law between shu (庶) (commoner) households and shi (士) (noble) households in the Nine ranks system which was created by Cao Wei and enabled hereditary officeholding by the aristocratic magnate families.

[citation needed] According to the Book of Wei by Wang Chen, the Cao family descended from the Yellow Emperor through his grandson Zhuanxu.

This account was attacked by Jiang Ji, who claimed that those with the family name "Tian" descended from Shun, but not those surnamed "Cao".