Isshi Incident

What Happened

On 10 July 645, at the Itabuki Palace, a group of conspirators led by Nakatomi no Kamatari and Prince Naka no Ōe assassinated Soga no Iruka, the most powerful figure at the Yamato court. The killing was carried out during a formal audience, when the prince and his allies struck Iruka down in front of the assembled courtiers. The plot did not stop with Iruka: the conspirators moved immediately against the main branch of the Soga clan, eliminating its leading members and dismantling its political dominance.

The incident takes its name from the zodiacal designation of the year 645, which was the Year of the Yin Wood Snake in the traditional East Asian sexagenary cycle. That same year, the victorious faction launched the Taika Reform, a sweeping programme of political and administrative change that reshaped the imperial state. The Isshi Incident is therefore understood as the opening act of the Taika era, clearing the way for the reforms that followed.

Background

By the early 640s, the Soga clan had held effective control over the Yamato court for decades. The main branch of the family had intermarried with the imperial line, installed and removed emperors at will, and monopolised the highest offices. Soga no Iruka, the head of the clan at the time, exercised power so openly that he was widely resented by rival noble houses and by members of the imperial family itself.

Prince Naka no Ōe, a son of Emperor Kōtoku's predecessor, saw the Soga dominance as a direct threat to the throne. He found a willing ally in Nakatomi no Kamatari, a court official from a family of Shinto ritualists who had long been sidelined by the Soga-backed Buddhist establishment. Together they recruited a small circle of conspirators, including figures close to the palace, and waited for the right moment to strike. The assassination at Itabuki Palace was the result of months of careful planning.

The plot was also a response to the wider political situation on the continent. The Tang dynasty in China had recently reunified East Asia, and its sophisticated administrative model was becoming a point of reference for reform-minded Japanese courtiers. The Soga clan's resistance to such reforms, and its insistence on maintaining its own power, made it an obstacle to the kind of centralised state that Naka no Ōe and Kamatari envisioned.

Why It Mattered

The Isshi Incident broke the Soga clan's grip on the Yamato court and restored effective authority to the imperial line. With the main branch of the clan eliminated, Prince Naka no Ōe and his allies were free to implement the Taika Reform, which introduced a system of provinces and districts, nationalised land ownership, and established a census and tax regime modelled on Tang practice. These reforms laid the administrative foundations of the Japanese state for centuries to come.

The incident also elevated Nakatomi no Kamatari to a position of lasting influence. His descendants, the Fujiwara clan, would go on to dominate Japanese court politics for the next several hundred years. Prince Naka no Ōe himself later ascended the throne as Emperor Tenji, and his reign is remembered as a turning point in the consolidation of imperial power.

The Name in Japanese

The Japanese name is 乙巳の変, read in kana as いっしのへん, and in romaji as Isshi no Hen.

The word 変 (へん, hen) means "incident" or "upheaval" — the same character used in the names of other political coups in Japanese history, such as the 本能寺の変 (Honnōji no Hen). It carries a sense of sudden, violent change rather than a gradual shift.

The first two characters together, 乙巳 (いっし, isshi), form the zodiacal name for the year 645. Each character is one half of the sexagenary cycle:

• 乙 (いつ, itsu) is the second of the ten Heavenly Stems. Its meaning in this context is "second" or "younger"; it is associated with the element wood and with the yin principle. In the cycle, it is paired with the second Earthly Branch to mark a specific year.
• 巳 (し, shi) is the sixth of the twelve Earthly Branches. It corresponds to the Snake in the Chinese zodiac, and in this word it simply names that position in the cycle.

So 乙巳 literally means "the second stem paired with the Snake," which is the way the year 645 was designated in the traditional calendar. The の (no) particle between 乙巳 and 変 marks possession or attribution, making the whole phrase "the incident of the Yin Wood Snake year." The reading isshi is a compressed pronunciation of いつ + し, where the final consonant of the first syllable merges with the following syllable — a common pattern in Japanese compound readings.

Key Figures

Nakatomi no Kamatari (later known as Fujiwara no Kamatari) was the chief architect of the plot. A court official from a family of Shinto ritualists, he recruited Prince Naka no Ōe and coordinated the conspiracy. After the incident, he became the most trusted adviser of the new regime, and his descendants founded the Fujiwara clan.

Prince Naka no Ōe was the imperial prince who carried out the assassination alongside Kamatari. He later reigned as Emperor Tenji, and his reign saw the implementation of the Taika Reform and the consolidation of the new administrative system.

Soga no Iruka was the head of the Soga main branch and the target of the assassination. His death at Itabuki Palace marked the end of his clan's dominance over the Yamato court.