Prior to his accession, he was known as Prince Naka-no-Ōe (中大兄皇子,Naka-no-Ōe no Ōji).
Events of Tenji's life
As prince, Naka no Ōe played a crucial role in ending the near-total control the Soga clan had over the imperial family. In 644, seeing the Soga continue to gain power, he conspired with Nakatomi no Kamatari and Soga no Kurayamada no Ishikawa no Maro to assassinate Soga no Iruka in what has come to be known as the Isshi Incident. Although the assassination did not go exactly as planned, Iruka was killed, and his father and predecessor, Soga no Emishi, committed suicide soon after. Following the Isshi Incident, Iruka's adherents dispersed largely without a fight, and Naka no Ōe was named heir apparent. He also married the daughter of his ally Soga no Kurayamada, thus ensuring that a significant portion of the Soga clan's power was on his side.
Events of Tenji's reign
Naka no Ōe reigned as Emperor Tenji from 661 to 672.
661: In the 3rd year of Saimei's reign (斉明天皇三年), the empress designated her son as her heir; and modern scholars construe this as meaning that this son would have received the succession (senso) after her death or abdication. Shortly after, she died, and Emperor Tenji could be said to have acceded to the throne (sokui).
662: Tenji is said to have compiled the first Japanese legal code known to modern historians. The Ōmi Code, consisting of 22 volumes, was promulgated in the last year of Tenji's reign. This legal codification is no longer extant, but it is said to have been refined in what is known as the Asuka Kiyomihara ritsu-ryō of 689; and these are understood to have been a forerunner of the Taihō ritsu-ryō of 701.
666: Tenji invaded Korea in support of Japan's ally Paekche but was seriously defeated at the battle of Battle of Baekgang by the combined forces of Silla and Tang China.
668: An account in Nihon Shoki becomes the first mention of petrochemical oil in Japan. In the 7th year of Tenji's reign (天智七年), flammable water (possibly petroleum)[not specific enough to verify] was presented as an offering to Emperor Tenji from Echigo Province (now known as a part of Niigata Prefecture). This presentation coincided with the emperor's ceremonial confirmation as emperor. He had postponed formalities during the period that the mausoleum of his mother was being constructed; and when the work was finished, he could delay no longer. Up until this time, although he had been de facto monarch, he had retained the title of Crown Prince.
671: An account in Nihon Shoki becomes the first mention of public announcement of time by rōkoku (a kind of water clock) in Japan. In 660 also a mention of this kind of clock exists.
Tenji was particularly active in improving the military institutions which had been established during the Taika Reforms.
Death of the emperor
Memorial Shinto shrine and mausoleum honoring Emperor Tenji
Following his death in 672, there ensued a succession dispute between his fourteen children (many by different mothers). In the end, he was succeeded by his son, Prince Ōtomo, also known as Emperor Kōbun, then by Tenji's brother Prince Ōama, also known as Emperor Tenmu. Almost one hundred years after Tenji's death, the throne passed to his grandson Emperor Kōnin.
In the 10th year of Tenji, in the 11th month (671): Emperor Tenji, in the 10th year of his reign (天智天皇十年), designated his son as his heir; and modern scholars construe this as meaning that the son would have received the succession (senso) after his father's death. Shortly thereafter, Emperor Kōbun is said to have acceded to the throne (sokui). If this understanding were valid, then it would follow:
In the 1st year of Kōbun (672): Emperor Kōbun, in the 1st year of his reign (弘文天皇一年), died; and his uncle Ōaomi-shinnō received the succession (senso) after the death of his nephew. Shortly thereafter, Emperor Tenmu could be said to have acceded to the throne (sokui).
Prior to the 19th century, Ōtomo was understood to have been a mere interloper, a pretender, an anomaly; and therefore, if that commonly accepted understanding were to have been valid, then it would have followed:
In the 10th year of Tenji, in the 11th month (671): Emperor Tenji, in the 10th year of his reign (天智天皇十年), died; and despite any military confrontations which ensued, the brother of the dead sovereign would have received the succession (senso); and after a time, it would have been understood that Emperor Tenmu rightfully acceded to the throne (sokui).
The Man'yōshū includes poems attributed to emperors and empresses; and according to Donald Keene, evolving Man'yōshū studies have affected the interpretation of even simple narratives like "The Three Hills." The poem was long considered to be about two male hills in a quarrel over a female hill, but scholars now consider that Kagu and Miminashi might be female hills in love with the same male hill, Unebi. This still-unresolved enigma in poetic form is said to have been composed by Emperor Tenji while he was still Crown Prince during the reign of Empress Saimei:
Prince Ōtomo (Ōtomo-shinnō) was the favorite son of Emperor Tenji; and he was also the first to be accorded the title of Daijō-daijin.
The years of Tenji's reign are not linked by scholars to any era or nengō. The Taika era innovation of naming time periods – nengō – languished until Mommu reasserted an imperial right by proclaiming the commencement of Taihō in 701.
In this context, Brown and Ishida's translation of Gukanshō offers an explanation about the years of Empress Jitō's reign which muddies a sense of easy clarity in the pre-Taiho time-frame:
"The eras that fell in this reign were: (1) the remaining seven years of Shuchō [(686+7=692?)]; and (2) Taika, which was four years long [695–698]. (The first year of this era was kinoto-hitsuji .) ... In the third year of the Taka era , Empress Jitō yielded the throne to the Crown Prince."
^Titsingh, p. 54; Varley, H. Paul. (1980). Jinnō Shōtōki, p. 44; a distinct act of senso is unrecognized prior to Emperor Tenji; and all sovereigns except Jitō, Yōzei, Go-Toba, and Fushimi have senso and sokui in the same year until the reign of Emperor Go-Murakami.