Rationale
It is conventionally regarded that the shogunate imposed and enforced the sakoku policy in order to remove the colonial and religious influence of primarily Spain and Portugal, which was perceived as posing a threat to the stability of the shogunate and to peace in the archipelago. The increasing number of Catholic converts in southern Japan (mainly Kyūshū) was a significant element of that which was seen as a threat. Based on work conducted by Japanese historians in the 1970s some scholars have however challenged this view, believing it to only be a partial explanation of political reality. The motivations for the gradual strengthening of the maritime prohibitions during the early 17th century should be considered within the context of the Tokugawa Bakufu's domestic agenda. One element of this agenda was to acquire sufficient control over Japan's foreign policy so to not only guarantee social peace, but to also maintain Tokugawa supremacy over the other powerful lords in the country, particularly the tozama daimyo. These daimyo had used East Asian trading linkages to profitable effect during the sengoku period, which allowed them to build up their military strength as well. By restricting the daimyo's ability to trade with foreign ships coming to Japan or pursue trade opportunities overseas, the Tokugawa bakufu could ensure none would become too powerful to challenge the bakufu's supremacy. This is consistent with the generally agreed rationale for the Tokugawa bakufu's implementation of the system of alternate attendance, or sankin kōtai. Directing trade predominantly through Nagasaki, which came under Toyotomi Hideyoshi's control in 1587, would enable the bakufu, through taxes and levies, to bolster its own treasury. This was no small matter as lack of wealth had limited both the preceding Kamakura bakufu and the Muromachi bakufu in crucial ways. The focus on the removal of Western and Christian influence from the Japanese archipelago as the main driver of the kaikin could be argued to be a somewhat eurocentric reading of Japanese history, although it is a common perception.
Nevertheless Christianity, and the two colonial powers it was most strongly associated with, were seen as genuine threats by the Tokugawa bakufu. Once the remnants of the Toyotomi clan had been defeated in 1615, Tokugawa Hidetada turned his attention to the sole remaining credible challenge to Tokugawa supremacy. Religious challenges to central authority were taken seriously by the bakufu as ecclesiastical challenges by armed Buddhist monks were common during the sengoku period. The Empress Meishō (1624–96) also had great doubts when she heard about how the Spanish and Portuguese were settling in the New World, and thought that Japan would soon become one of the many countries in their possession.
Protestant English and Dutch traders reinforced this perception by accusing the Spanish and Portuguese missionaries of spreading the religion systematically, as part of a claimed policy of culturally dominating and colonizing Asian countries. The Dutch and English were generally seen by the Japanese to be able to separate religion and trade, while their Iberian counterparts were looked upon with much suspicion. The Dutch, eager to take over trade from the Spanish and Portuguese, had no problems reinforcing this view. The number of Christians in Japan had been steadily rising due to the efforts of missionaries, such as Francis Xavier and daimyo converts. The direct trigger which is said to have spurred the imposition of sakoku was the Shimabara Rebellion of 1637–38, an uprising of 40,000 mostly Christian peasants. In the aftermath, the shogunate accused missionaries of instigating the rebellion, expelled them from the country, and strictly banned the religion on penalty of death. The remaining Japanese Christians, mostly in Nagasaki, formed underground communities and came to be called Kakure Kirishitan. All contact with the outside world became strictly regulated by the shogunate, or by the domains (Tsushima, Matsumae, and Satsuma) assigned to the task. Dutch traders were permitted to continue commerce in Japan only by agreeing not to engage in missionary activities. Today, the Christian percentage of the population (1%) in Japan remains far lower than in other East Asian countries like China (5%), Vietnam (7%), South Korea (29.2%) and the Philippines (over 90%).
The sakoku policy was also a way of controlling commerce between Japan and other nations, as well as asserting its new place in the East Asian hierarchy. The Tokugawa had set out to create their own small scale international system where Japan could continue to access the trade in essential commodities such as medicines, and gain access to essential intelligence about happenings in China, while avoiding having to agree to a subordinate status within the Chinese tributary system. Japan's generally constructive official diplomatic relationship with Joseon Korea allowed regular embassies (Tongsinsa) to be dispatched by Korea to Japan. Together with the brisk trade between Tsushima and Korea, as well as the presence of Japanese in Pusan, Japan was able to access Chinese cultural, intellectual and technological developments throughout the Edo period. At the time of the promulgation of the strictest versions of the maritime prohibitions, the Ming dynasty had lost control of much of China and it was unnecessary, and perhaps undesirable, for Japan to pursue official diplomatic relations with either of the Ming or the Qing governments while the issue of imperial legitimacy was unsettled. Japan was able to acquire the imported goods it required through intermediary trade with the Dutch and through the Ryukyu Islands. The Japanese actually encouraged the Ryūkyū Kingdom's rulers to maintain a tributary relationship with China, even though the Shimazu clan had surreptitiously established great political influence in the Ryukyu Islands. The Qing became much more open to trade after it had defeated the Ming loyalists in Taiwan, and thus Japan's rulers felt even less need to establish official relations with China.
Liberalizing challenges to sakoku did come from within Japan's elite in the 18th century, but they came to naught. Later on, the sakoku policy was the main safeguard against the total depletion of Japanese mineral resources — such as silver and copper — to the outside world. However, while silver exportation through Nagasaki was controlled by the shogunate to the point of stopping all exportation, the exportation of silver through Korea continued in relatively high quantities.
The way Japan kept abreast of Western technology during this period was by studying medical and other texts in the Dutch language obtained through Dejima. This developed into a blossoming field in the late 18th century which was known as Rangaku (Dutch studies). It became obsolete after the country was opened and the sakoku policy collapsed. Thereafter, many Japanese students (e.g. Kikuchi Dairoku) were sent to study in foreign countries, and many foreign employees were employed in Japan (see o-yatoi gaikokujin).
The policies associated with sakoku ended with the Convention of Kanagawa in response to demands made by Commodore Perry.
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