Clan Adachi was destroyed in the 1285 Shimotsuki Incident. The purge was driven by a power struggle inside the Kamakura shogunate: Taira Yoritsuna, the Hōjō family’s house steward, moved against Adachi Yasumori and his supporters, and the Adachi lost their position as one of the most powerful rival factions. The broad political explanation is well supported by the sources available. The exact accusation that set the purge in motion, the sequence of orders, and the personal motives of the main figures are not settled.
What the Shimotsuki Incident was
When people ask about the destruction of Clan Adachi, they usually mean the Shimotsuki Incident, named for the eleventh lunar month of 1285. (The sources give the coup’s date as November 1285 in the Gregorian calendar, which is why the two dates appear side by side.) Adachi Yasumori was a senior figure in the Kamakura shogunate and a powerful ally of the Hōjō family, which held the regency and dominated the government.
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The Kanagawa Prefectural Archives’ overview of Kamakura as Japan’s warrior capital states that Taira Yoritsuna falsely accused Yasumori, and that this accusation led to the destruction of Yasumori and his faction. The same overview describes Yasumori and allied warriors dying in the purge, and it lists several other families whose members took their own lives alongside him. It does not establish a reliable total death count, so no exact figure should be given for the number of people killed.
Why it happened: the competing interests
The most useful explanation is political rivalry inside the bakufu, the shogunal government, rather than a simple family feud. A scholarly article abstract published through J-STAGE, the Japanese academic platform, frames the conflict as two opposing interests. One was an administrator-oriented group represented by Yasumori. The other was a group concerned with protecting the interests of the shogunate’s warrior vassals, represented by Yoritsuna. The abstract connects this split to governance, debt remission, and the changing role of the shogunate itself.
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A catalog record at the National Diet Library summarizes a different scholarly interpretation. It holds that the Hōjō Tokusō government adopted policies after the Mongol invasions, and that the political movement tied to those policies was disrupted by the 1285 coup in which Yasumori was killed. This is a broad interpretation of the period, not evidence that one specific policy dispute caused the purge.
Three ways to read the purge
The sources do not support a single agreed cause. They point to three different layers of explanation, with different levels of certainty.
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| Explanation | What the source says | How much it supports |
|---|---|---|
| Personal accusation | Yoritsuna falsely accused Yasumori, and the accusation led to the purge (Kanagawa Prefectural Archives overview). | Strong on the outcome and the role of Yoritsuna. The text of the accusation is not established in the sources. |
| Factional rivalry over governance | Yasumori represented an administrator-oriented approach, while Yoritsuna represented the interests of warrior vassals (J-STAGE article abstract, “A Reexamination of the Shimotsuki Incident,” Shigaku Zasshi). | Good support for the general division of interests. The full article argument was not available, so its wider conclusions cannot be stated. |
| Post-Mongol policy disruption | The coup disrupted a political movement linked to policies adopted after the Mongol invasions (National Diet Library catalog summary of a scholarly work). | Moderate support as a historical interpretation. It describes context and consequences more than a proven trigger. |
What is still uncertain
The sources do not settle the exact accusation against Yasumori, the full sequence of orders, or the personal motives of the principal actors. Japanese Wiki Corpus, a secondary online overview, describes the Shimotsuki Incident note as the only primary archive for the incident. That claim has not been checked against a critical edition or the medieval record itself, so it should not be presented as settled.
The historiography is also contested. The ‘Reexamination’ article exists precisely because the conventional account of late Kamakura politics is being revisited, and its conclusions are not available in the abstract alone. Readers should treat the rivalry explanation as the best-supported reading, not a final verdict.
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What to say, and what to avoid
- Safe to state: the purge was the 1285 Shimotsuki Incident, Yoritsuna moved against Yasumori, and the Adachi faction was destroyed in a contest over political authority in the shogunate.
- Needs a qualifier: the role of the vassals’ interests and debt remission, which come from a single abstract, and the post-Mongol policy link, which comes from a catalog summary.
- Do not state as proven: a specific plot, an explicit order from Hōjō Sadatoki, a particular reform as the trigger, or any exact death toll.
Search phrasing such as “Why was Adachi Yasumori killed?” points to the same event. Yasumori was the family’s leader, so the answer for both is the Shimotsuki Incident.
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Sources and confidence
- High confidence for the broad outline: the Kanagawa Prefectural Archives overview, “I. Kamakura: Warrior Capital of Japan,” for the purge, the accusation by Yoritsuna, and the destruction of Yasumori’s faction.
- High confidence for the abstract’s wording only: “A Reexamination of the Shimotsuki Incident,” Shigaku Zasshi, via J-STAGE.
- Moderate confidence: the National Diet Library catalog summary of a scholarly work on the Mongol invasions, the Hōjō Tokusō government, and the 1285 coup.
- Cautious secondary context only: Japanese Wiki Corpus, whose claimed primary-source basis has not been independently verified.
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