Emperor Jin Wudi 晉武帝 (236-290, r. 265-290), personal name Sima Yan, courtesy name Anshi 安世, was the founder of the Jin dynasty 晉 (265-420). He was a son of Sima Zhao 司馬昭 (211-265) and a grandson of Sima Yi 司馬懿 (179-251), a powerful minister of the Wei empire 曹魏 (220-265) of the Three Kingdoms 三國 (220~280 CE). The family hailed from Wenxian 溫縣 in the commandery of Henei 河內 (present-day Wenxian, Henan). During the Jiaping reign-period 嘉平 (249-253), Sima Yi was bestowed the title of Neighbourhood Marquis of Beiping 北平亭侯, later on Township Marquis of Xinchang 新昌鄉侯. He also bore the title of General of the Central Bulwark (zhonglei jiangjun 中壘將軍).
On the throne accession of Emperor Yuan 魏元帝 (r. 260-265) of the Wei dynasty, Sima Zhao was made regent. In 263, he adopted the title of Prince of Jin 晉王 and gave his oldest son, Sima Yan, the right of inheritance of his titles of nobility and his office of Counsellor-in-chief (xiangguo 相國).
In 265, after the death of his father, Sima Yan forced Emperor Yuan to resign (shanwei 禪位) and proclaimed himself emperor of the Jin dynasty, his residence being in Luoyang 洛陽 (today in Henan). According to custom, Sima Yan founded an ancestral shrine for the imperial family (zongshi 宗室). Sima Yi was posthumously given the dynastic title of Emperor Xuan 晉宣帝, Sima Yan's uncle Sima Shi 司馬師 (208-255) that of Emperor Jing 晉景帝, and his father Sima Zhao that of Emperor Wen 晉文帝. Sima Yan's empress was Yang Yan 楊艷. His son Sima Zhong 司馬衷 was given the title of Heir Apparent (huang taizi 皇太子).
Sima Yan's first reign motto was Taishi 泰始 (265-274), and he changed the astronomical calculations of the Jingchu Calendar 景初歷 to that of the Taishi Calendar 泰始歷 (see calendar).
In 280, Sima Yan destroyed the empire of Wu 吳 (222-280) in southeast China and so reunited China.
Sima Yan initiated in 266 a reform policy concerning taxation and local administration. He abolished the military units in the provinces and commanderies (zhoujun bing 州郡兵) and the military agro-colonies (tuntian 屯田) and ordered the administrators of commanderies and districts to tax agricultural land and mulberry plantations. The military consisted of troops drafted from the peasant population according to the system of labour corvée (yaoyi 徭役). Farmers of agro-colonies were transformed into regularly registered households (bianhu 編戶) which had to pay taxes. The government encouraged agricultural production and forbade the merchandise of grain. It also urged parents of unmarried girls older than 17 sui to marry them, in order to increase the population.
The new taxation system consisted of three parts, namely a "field-occupation law" (zhantianfa 占田法), a "land taxation system" (ketianfa 課田法, see zhantian ketian zhi 占田課田制), and the household taxation law (hudiaofa 戶調法). With these methods, the improvised quasi-military administration of the Wei period was abolished and the national economy quickly recovered. At the same time, the state revenue increased. Taxes were not collected by petty officials (limin 吏民), but by the most powerful families in the commanderies, or the imperial princes. Sima Yan hoped that in this way, the local government would strengthen the central government.
On the other hand, the field occupation or field ownership law strengthened the privileges of the eminent families (menfa 門閥, shizu 世族) by allowing them to own large tracts of tax-exempted land, house a certain number of relatives in their household (which were therefore not liable for taxation), keep large numbers of client-farmers (dianke 佃客) and dependants (yinke 蔭客) on the fields and in the household (yishike 衣食客), and to rely on private armies (buqu 部曲) for self-defense. The size of fields and the number of servants depended on the classification of the family according to the nine-rank system (jiupin zhongzheng zhi 九品中正制) introduced by the Wei dynasty.
The classification system also pertained to the appointment to state office, so that poor men were barely able to rise to high offices, and no scion of a wealthy family would be in a low position (shangpin wu hanmen, xiapin wu shizu 上品無寒門，下品無世族).
Another measure to revive the economy of north China was to invite farmers of the Sichuan Basin (the former empire of Shu 蜀漢, 221-263) and southeast China (former Wu empire) to settle down on free land in the north. In order to promote such resettlements, the Jin court promised exemption from labour corvée for twenty years. The measure proved effectual: In 283, the number of registered households ran up to 3.7 million, which was fifty per cent more than before.
Sima Yan also invited members of influential families from the former empires of Shu and Wu to serve the court in Luoyang.
In the early years of his reign, Sima Yan was known as a ruler who "diligently observed benevolence and austerity" (jiao yi ren jian 矯以仁儉). He presented the orphaned and poor with extra-rations of grain and ordered the commandery governors to see to it that no one was suffering. He also urged regional inspectors to deliver reports to him telling the truth about the situation. Sima Yan strictly kept to his new law code Jinlü 晉律, issued in 268, and often judged in person according to the new laws. The new law code was more concise than that of the Han dynasty 漢 (206 BCE-220 CE) and focused on an elaborate method of penal law. The code was distributed in all districts and villages so that everyone was informed about crimes and consequences.
Yet after the downfall of the empire of Wu, this attitude changed, and dissipation at the court became common. Sima Yan equipped twenty-seven of his relatives with large estates in princedoms with a size of commanderies. He abolished the custom of the Han and Wei dynasties to bestow vain titles of prince (wang 王) and marquis (hou 侯) without giving the holders land. Instead, he desired to reintroduce the system of regional rulers ("feudal system") of the Zhou period 周 (11th cent.-221 BCE) and ordered the princes to reside in their princedoms, and not in the imperial capital. In this way, the many princes were "vice-regents" of the emperor and wielded particular autonomy in their princedoms. The founder of the dynasty thus not only contributed to the prodigious extravagance of the many princes, but also to their economic, military and political power, which was a critical factor for the downfall of the Western Jin. The larger princedoms were allowed an army of no less than 5,000 troops, mid-size princedoms 3,000 men, and the smaller ones 1,100 troops.
The emperor was also known for his indulgence in the pleasures of the "harem", and seized all 5,000 ladies at the court of the former Wu empire for his own harem, which included "nearly 10,000 females". In 273, Sima Yan even ordered to send selected daughters from all social classes throughout the empire (barring those of the nobility) to the imperial household and stopped marriages until this recruitment process was finished.
Shortly before his death, Sima Yan appointed his father-in-law Yang Jun 楊駿 (d. 291) Defender-in-chief (taiwei 太尉), supreme commander of the armies (dudu zhongwai zhujun shi 都督中外諸軍事), and made him regent, together with his uncle Sima Liang 司馬亮 (d. 291), Prince of Runan 汝南. Sima Yan died in 290 and was buried in the tomb mound Junyangling 峻陽陵 close to Luoyang. His posthumous title is "Martial Emperor" (Wudi 武帝), and his dynastic title Shizu 世祖.
Emperor Wu was succeeded by his second son Sima Zhong 司馬衷, who is known as Emperor Hui 晉惠帝 (r. 290-306).