It is probably more appropriate to rephrase this question into “When did the Kamakura period begin?”

Japan uses two periodization schemes, one being pretty much equivalent to standard European periodization (Ancient, Classical, [Early Modern,] Modern), the other being principally based on political change. Here, the naming convention is centered on the perceived political center(s) of the respective period: the imperial capitals of Nara and Heian and the seats of the shogunates (Kamakura, Muromachi, Edo). In the last decades, separating the Nanbokuchō and Sengoku periods (two, or no single political center respectively) clearly from Muromachi became widespread.

Theoretical Concerns

The question that therefore needs to be debated to define the beginning and end of such a period is: when can we speak of X as the political center? The debate therefore inevitably has to center on questions of political theory, specifically of the theory of state. More precisely, on theory of premodern—or rather: pre-nation-state—states.

This raises fundamental questions: when is a state a state? What are the sufficient, and what are the necessary conditions for any sociopolitical entity to be considered a distinct state, or state-like entity?

This is the reason why the beginning of the Kamakura period is one of the most difficult questions in Japanese periodization. The choice of referring to Yoritomo’s seat of power as the namesake for the period (despite the imperial court being by no means powerless) may be a remnant of a historiographical tradition that saw a revolutionary moment in the emergence of warrior government. For modern historians following the landmark work of Hara Katsurō, this emergence was tantamount to what Nitta Ichirō discusses as the discovery of Europe within Japan’s past (1).

Still, we are now stuck with the arrangement and need to answer the question when we can speak of Yoritomo’s “government” as a state-like entity possessing both the sufficient and the necessary characteristics—whatever those may be!—to define the point of transition from one historical period to another. The central issues here are encapsulated in discourses of warrior government itself, embedded in the older problem of Japanese feudalism, which defined the Kamakura bakufu as a feudal state, and the idea of an Eastern State (Tōgoku kokka ron) that was first proposed in the middle of the 20th century (2). To make matters worse, in 1963 Kuroda Toshio proposed a very influential model of the medieval state as a compound state nominally headed by the Tenno, but in fact being composed of multiple, semi-independent entities that mutually complemented each other known as kenmon taisei ron. The (never-ending) historiographical debate has since oscillated between these three frameworks, with the main trajectory since the 1960s being focused on leaning more towards either Kuroda or Satō, or attempting a haphazard synthesis of the two approaches (3).

Proposed Starting Years

The main years that have been discussed in the debates on the beginning of the Kamakura period are 1180, 1183, 1185, 1190, and 1192 (4).

If we would speak of the Kamakura-polity’s historical self-perception, the answer would be quite easy: when Yoritomo decided to raise in arms against the Taira in 1180 (this is why the Azuma kagami, which purports to tell the story of Kamakura rule, begins with this event) and, a couple months later, enters Kamakura, henceforth his seat of operations on the tenth month, sixth day of 1180.

But historians such as Kondō Shigekazu, for example, endorse the year 1183 for the following reason: this was the year when the Taira government became unable to hold control over Kyoto, and when the imperial court declared Yoritomo the rightful military leader of the imperial cause, legitimizing the rebellious government he had build for the past three years in the East, which can be interpreted as the emergence of a legitimate Eastern state (5).

In contrast, the contemporary default position taught in schools, 1185, emphasizes the importance of the 1185 imperial decree that invested Yoritomo with Japan-wide authority to appoint shugo and jitō in the provinces and estate lands, elevating his government beyond the geographical limitations of the East to an entity with geographically unrestricted “public” authority that is nominally subjugated to—but also legitimized by—the imperial court. (The problem with this 1185-decree is that it can be evaluated as a temporary measure to restore law and order caused by the Genpei War, not as a permanent arrangement.)

A critique of these approaches can be made because, if we only talk about the expansion of Yoritomo’s authority through imperial decree, several interim cases might also warrant discussion: for example, in the first month of 1184, a decree issues the elimination of Kiso Yoshinaka, Yoritomo’s only real rival, thus leaving Yoritomo as the sole legitimate military representant of the imperial court in the conflict against the Taira. Several other decrees gradually expand Yoritomo’s authority over the next two years (6). (In other words: why 1183 and 1185 are more important than other decrees is a problem that needs to be addressed by historians concerned with this problem.)

Moreover, an argument was made for Yoritomo’s appointment to high nobility and his investure as nihonkoku sōtsuibushi [Supreme Constable of Japan] in 1190, which establishes him as the nominal protector of all of Japan beyond the military emergency measures of the 1180s. Likewise, the appointment to seii taishōgun of 1192 would provide a similar angle of argument: in both cases, the emphasis is on Yoritomo’s role as military overlord within the framework of the Japanese imperial state (7). (As I see it, the only reason why anyone would even endorse 1192, however, is by overemphasizing what the position of shogun would come to signify after Yoritomo's death and projecting this significance back in time.)

Conclusion

As can be seen, the debate incorporates on several aspects: the military function, the (territorial and/or ideological) independence of the East, and the diplomatic relations between of court and shogunate and the corresponding question of legitimacy. Whereas I'm not making Japanese schoolbooks and thus can't explain the reasoning that actually is behind the choice of publicly endorsed narratives, I would suggest that the 1185 choice strikes a balance between a nominal Japan-wide authority and the aura of legitimacy obtained through the imperial court, although academic historians remains skeptical, since there is room for debate on what this decree ultimately means.

Notes

(1) Hara Katsurō, Nihon chūseishi (Tokyo: Fuzanbō, 1906). Reprinted countless times by various publishers. See also Nitta Ichirō, Chūsei ni kokka wa atta ka (Tokyo: Yamakawa Shuppansha, 2004), for an explication of the entanglement of discourses of feudalism, the idea of the medieval, and the role of history in the self-perception of the modern Japanese state.

(2) cf. Kondō Shigekazu, Kamakura jidai seiji kōzō no kenkyū (Tokyo: Azukura Shobō, 2016a), 35–39.

(3) A summary of this principal debate can be found in Kondō Shigekazu, “The Horse-Race for the Throne: Court, Shogunate, and Imperial Succession in Early Medieval Japan,” doi.org/10.14220/9783737012416.105. A pre-submission manuscript is available on the translator’s (I wonder who that might be?) academia account.

(4) cf. Takahashi Noriyuki, “Kamakura bakufu ron,” in Iwanami kōza Nihon rekishi, vol. 6: Chūsei 1 (Tokyo: Iwanami Shoten, 2013), 97–101.

(5) Kondō Shigekazu, Kamakura bakufu to chōtei (Tokyo: Iwanami Shoten, 2016b), 36. 1183 was first endorsed in Satō Shin’ichi, Kamakura bakufu soshō seido no kenkyū (Tokyo: Iwanami Shoten, 1943).

(6) Gomi Fumihiko, Kamakura jidai ron (Tokyo: Yoshikawa Kōbunkan, 2020), 20–22. Gomi indeed identifies ten decrees between the tenth month of 1183 and the tenth month of 1186 which in some way may have played a larger or smaller role in this process.

(7) As pointed out by Takahashi 2013, 100.