Early reign: establishment of the Jin political system

Emperor Wu immediately sought to avoid what he saw as Cao Wei's fatal weakness—lack of power among the imperial princes. In 265, immediately after taking the throne, he made princes of many of his uncles, cousins, brothers, and sons, each with independent military commands and full authority within their principalities. This system, while it would be scaled back after the War of the Eight Princes and the loss of northern China, would remain in place as a Jin institution for the duration of the dynasty's existence, and would be adopted by the succeeding Southern dynasties as well.

Another problem that Emperor Wu saw with Cao Wei's political system was its harshness in penal law, and he sought to reform the penal system to make it more merciful—but the key beneficiaries of his changes turned out to be the nobles, as it quickly became clear that the mercy was being dealt out in an unequal manner. Nobles who committed crimes often received simple rebukes, while there were no meaningful reductions in penalties for commoners. This led to massive corruption and extravagant living by the nobles, while the poor went without government assistance. For example, in 267, when several high level officials were found to have worked in conjunction with a county magistrate to seize public land for themselves, Emperor Wu refused to punish the high level officials while punishing the county magistrate harshly.

Emperor Wu faced two major military issues almost immediately—incessant harassment from the rival Eastern Wu's forces, under emperor Sun Hao, and tribal rebellions in Qin (秦) and Liang (涼) provinces (modern Gansu). Most officials were more concerned about the Xianbei, Qiang and other tribes in northwestern China and also with the Xiongnu in modern Shanxi, who were resettled there after the dissolution of their state by Cao Cao in 216 under the watchful eyes of Chinese officials, and were feared for their military abilities. These officials advised Emperor Wu to try to suppress the tribes before considering conquests of Eastern Wu. Under the encouragement of the generals Yang Hu and Wang Jun and the strategist Zhang Hua, however, Emperor Wu, while sending a number of generals to combat the tribes, prepared the southern and eastern border regions for war against Eastern Wu throughout this part of his reign. He was particularly encouraged by reports of Sun Hao's cruelty and ineptitude in governing Eastern Wu; indeed, the officials in favor of war against Eastern Wu often cited this as reason to act quickly, as they argued that Eastern Wu would be harder to conquer if and when Sun Hao was replaced. However, after a major revolt by the Xianbei chief Tufa Shujineng started in 270 in Qin Province, Emperor Wu's attention became concentrated on Tufa, as Tufa was able to win victory after victory over Jin generals. In 271, the Xiongnu noble Liu Meng rebelled in Bing province as well, and while his rebellion did not last long, this took Emperor Wu's attention away from Eastern Wu. In 271, Jiao Province (交州, modern northern Vietnam), which had paid allegiance to Jin ever since the start of his reign, was recaptured by Eastern Wu. In 272, the Eastern Wu general Bu Chan, in fear that Sun Hao was going to punish him on the basis of false reports against him, tried to surrender the important city of Xiling (西陵, in modern Yichang, Hubei) to Jin, but Jin relief forces were stopped by the Eastern Wu general Lu Kang, who then recaptured Xiling and killed Bu. In light of these failures, Yang took another tack—he started a détente with Lu and treated the Eastern Wu border residents well, causing them to view Jin favorably.

When Emperor Wu ascended the throne in 265, he honored his mother Wang Yuanji as empress dowager. In 266, he also honored his aunt Yang Huiyu (Sima Shi's wife) an empress dowager, in recognition of his uncle's contributions to the establishment of the Jìn Dynasty. He made his wife Yang Yan empress the same year. In 267, he made her oldest living son, Sima Zhong crown prince—based on the Confucian principle that the oldest son by an emperor's wife should inherit the throne—a selection that would, however, eventually contribute greatly to political instability and the Jin Dynasty's decline, as Crown Prince Zhong appeared to be developmentally disabled and unable to learn the important skills necessary to govern. Emperor Wu further made perhaps a particularly fateful choice on Crown Prince Zhong's behalf—in 272, he selected Jia Nanfeng, the strong-willed daughter of the noble Jia Chong, to be Crown Prince Zhong's princess. Crown Princess Jia would, from that point on, have the crown prince under her own tight control. Before Empress Yang died in 274, she was concerned that whoever the new empress would be, she would have ambitions to replace the crown prince, and therefore Empress Yang asked Emperor Wu to marry her cousin Yang Zhi. He agreed.

In 273, Emperor Wu would undertake a selection of beautiful women from throughout the empire—a warning sign of what would eventually come. He looked most attentively at the daughters of high officials, but he also ordered that no marriages take place across the empire until the selection process was done.

Middle reign: unification of the Chinese empire

In 276, Emperor Wu suffered a major illness—which led to a succession crisis. Crown Prince Zhong would be the legitimate heir, but both the officials and the people hoped that Emperor Wu's capable brother, Sima You, the Prince of Qi, would inherit the throne instead. After Emperor Wu became well, he divested some military commands from officials that he thought favored Prince You, but otherwise took no other punitive actions against anyone.

Later that year, Yang Hu reminded Emperor Wu of his plan to conquer Eastern Wu. Most of the officials, still concerned with Tufa's rebellion, were opposed, but Yang was supported by Du Yu and Zhang. Emperor Wu considered their counsel seriously but did not implement it at this time.

Also in 276, pursuant to his promise to the deceased Empress Yang, Emperor Wu married his cousin Yang Zhi and made her empress. The new Empress Yang's father, Yang Jun, became a key official in the administration and became exceedingly arrogant.

In 279, with the general Ma Long having finally put down Tufa's rebellion, Emperor Wu concentrated his efforts on Eastern Wu, and commissioned a six-pronged attack led by his uncle Sima Zhou, Wang Hun, Wang Rong, Hu Fen (胡奮), Du Yu, and Wang Jun, with the largest forces under Wang Hun and Wang Jun. Each of the Jin forces advanced quickly and captured the border cities that they were targeting, with Wang Jun's fleet heading east down the Yangtze and clearing the river of Eastern Wu fleets. The Eastern Wu chancellor Zhang Ti (張悌) made a last-ditch attempt to defeat Wang Hun's force, but was defeated and killed. Wang Hun, Wang Jun, and Sima Zhou each headed for Jianye, and Sun Hao was forced to surrender in spring 280. Emperor Wǔ made Sun Hao the Marquess of Guiming. The integration of former Eastern Wu territory into Jin appeared to have been a relatively smooth process.

After the fall of Eastern Wu, Emperor Wu ordered that provincial governors no longer be in charge of military matters and become purely civilian governors, and that regional militias be disbanded, despite opposition by the general Tao Huang and the key official Shan Tao. This would also eventually prove to create problems later on during the upheaval of the Five Barbarians, as the regional governors were not able to raise troops to resist quickly enough. He also rejected advice to have the non-Han gradually moved outside of the empire proper.

Late reign: setting the stage for disasters

In 281, Emperor Wu took 5,000 women from Sun Hao's palace into his own, and thereafter became even more concentrated on feasting and enjoying the women, rather than on important matters of state. It was said that there were so many beautiful women in the palace that he did not know whom he should have sexual relations with; he therefore rode on a small cart drawn by goats, and wherever the goats would stop, he would stop there, as well.^[6] Because of this, many of the women planted bamboo leaves and salt outside their bedrooms—both items said to be favored by goats. Empress Yang's father Yang Jun and uncles Yang Yao (楊珧) and Yang Ji (楊濟) became effectively in power.

Emperor Wu also became more concerned about whether his brother Prince You would seize the throne if he died. In 282, he sent Prince You to his principality, even though there was no evidence that Prince You had such ambitions. Princess Jingzhao and Princess Changshan kow-towed and begged Emperor Wu to rescind his order, but he merely grew angry and demoted Princess Changshan's husband in retaliation.^[7] Prince You, in anger, grew ill and died in 283.

Following previous Roman embassies in 166 and 226, the Book of Jin and Wenxian Tongkao record another embassy from "Da Qin" appearing in China during the reign of Emperor Wu. These histories assert that it arrived in 284 and presented tributary gifts to the emperor.^[8][9]

As Emperor Wu grew ill in 289, he considered whom to make regent. He considered both Yang Jun and his uncle Sima Liang the Prince of Ru'nan, the most respected of the imperial princes. As a result, Yang Jun became fearful of Sima Liang and had him posted to the key city of Xuchang. Several other imperial princes were also posted to other key cities in the empire. By 290, Emperor Wu resolved to let Yang and Sima Liang both be regents, but after he wrote his will, the will was seized by Yang Jun, who instead had another will promulgated in which Yang alone was named regent. Emperor Wu died soon thereafter, leaving the empire in the hands of a developmentally disabled son and nobles intent on shedding each other's blood for power, and while he would not see the disastrous consequences himself, the consequences would soon come.