Prasat Thong

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  • Other Names/Titles: Phya Sriworawong, Phra Ong Lai, Chao Phya Kalahom Suryawong

Prasat Thong was king of the Siamese kingdom of Ayutthaya from 1629 to 1656. His rise to the throne, as the result of a violent coup, saw the destruction of the Japantown in Ayutthaya's capital, and the Tokugawa shogunate's severing of formal diplomatic relations with the kingdom.

Prasat Thong was born of royal blood, sometime around 1600. According to some accounts, he was the product of a dalliance by King Ekathotsarot with a village girl. Despite getting in trouble with the law several times as a youth, Prasat Thong rose by 1628 to a powerful and trusted position within the court of King Song Tham. In that year, as Song Tham was on his deathbed, he named Prasat Thong regent, and entrusted him and Yamada Nagamasa (head of the local Japanese community) with ensuring the peaceful succession of his son to the throne. Song Tham was indeed peacefully succeeded by his fifteen-year-old son, an apprentice Buddhist monk who took the throne in 1629 as King Cetthathirat. Shortly afterwards, Prasat Thong was promoted to the rank/title of Chao Phya Kalahom Suryawong; as a result, many of the European-language primary sources from the time refer to him as "the Kalahom."

The Kalahom desired the throne for himself, however. His two chief obstacles were Yamada, and Sri Sin, a brother of the former king. The Kalahom expertly pitted the two men against one another, convincing Yamada that it was Sri Sin who was plotting a coup; this led to the destruction of Sri Sin and his followers at the hands of Yamada's hundreds of Japanese warriors. King Cetthathirat was killed as well, before he had even been on the throne a full year; the circumstances of his death are left unclear by historian Cesare Polenghi. The Kalahom remained regent, reportedly with Nagamasa's agreement, as Song Tham's younger son now took the throne as the ten-year-old King Athittayawong.

Prasat Thong still saw Nagamasa as a threat. Since he could not have him killed in the capital without risking retribution from the Japanese royal guard, some 700 or so strong, he instead attempted to rid himself of Nagamasa by appointing him to a distant province. He called Nagamasa to court, and named him king of Ligor, a semi-independent kingdom to the south which had paid tribute to Ayutthaya since the 16th century (Ligor is today Thailand's Nakhon Si Thammarat province). In order to claim his throne, however, Nagamasa had to eliminate the previous king of Ligor, a "rebel" in the eyes of the Ayutthaya court, since he refused to pay tribute. Nagamasa marched down to Ligor in summer 1629, and secured his throne by the beginning of the next year, with the help of some 300 Japanese and several thousand Siamese troops. It is unclear whether the previous "rebel" king of Ligor was killed, sent to Ayutthaya as a prisoner, or kept on as an advisor to Nagamasa. In the meantime, with Nagamasa absent, Prasat Thong had the young Athittayawong killed, and claimed the throne of Ayutthaya for himself. Prasat Thong perhaps hoped that Nagamasa would be killed in battle in Ligor, thus ridding him of Nagamasa once and for all; though this did not take place, he found an opportunity soon enough. Nagamasa led an attack on the neighboring kingdom of Pattani, failing to subjugate it to Ligor's authority, and suffering a serious wound in the process. Prasat Thong then sent him ointments, as a gift, along with a number of Nagamasa's favorite women from the Ayutthaya royal harem, to serve him in his new court. However, according to most accounts, the ointment was poisoned, and it was by this, possibly with one of the court ladies aiding in the scheme, that Nagamasa was killed, eliminating the last major obstacle to Prasat Thong's reign.

When news reached Japan of this coup, however, the Tokugawa shogunate severed all ties with Ayutthaya, calling Prasat Thong a usurper. It must be remembered that in Japan, no one had ever dared to usurp the imperial throne, changing the dynasty, and also that at this time, in 1630, the Tokugawa were themselves in the process of establishing the stability of their own succession, having gained power less than 30 years earlier. So the issue of legitimacy, stability, and rightfulness of succession, or quite simply matters of principle in general, were of great importance to the Tokugawa. Prasat Thong sent several missions to Japan seeking to restore relations, but all were rebuffed.

Preceded by:
King of Ayutthaya
Succeeded by:
Chao Fa Chai


  • Cesare Polenghi, Samurai of Ayutthaya: Yamada Nagamasa, Japanese warrior and merchant in early seventeenth-century Siam. Bangkok: White Lotus Press (2009), 52-58.