Imagawa Sadayo (今川 貞世, 1326 - 1420), also known as Imagawa Ryōshun (今川 了俊), was a renowned Japanese poet and military commander who served as tandai ("constable") of Kyūshū under the Ashikaga bakufu from 1371 to 1395. His father, Imagawa Norikuni, had been a supporter of the first Ashikaga shogun, Ashikaga Takauji, and for his services had been granted the position of constable of Suruga province (modern-day Shizuoka prefecture). This promotion increased the prestige of the Imagawa family (a warrior family dating from the Muromachi period, which was related by blood to the Ashikaga shoguns) considerably, and they remained an important family through to the Edo period.
Sadayo's early life
During his early years Sadayo was taught Buddhism, Confucianism and Chinese, archery, and the military arts such as strategy and horse-back riding by his father (governor of the Tōkaidō provinces Tōtōmi and Suruga), along with poetry, which was to become one of his greatest passions. In his twenties he studied under Tamemoto of the Kyogoku school of poetry, and Reizei Tamehide of the Reizei school. At some point, he was appointed to head the boards of retainers and coadjudicators. He had taken religious vows when the Ashikaga bakufu called upon him to travel to Kyūshū and assume the post of constable of the region in 1370 after the failure of the previous constable to quell the rebel uprisings in the region, largely consisting of partisans of the Souther Court supporting one of the rebellious Emperor Go-Daigo's sons, Prince Kanenaga. By 1374-1375, Sadayo had crushed the rebellion, securing for the Bakufu northern Kyūshū, and ensuring the eventual failure of the rebellion and the consequent success of the Bakufu Shogunate. 
Kyūshū Tandai (1371-1395)
Sadayo's skill as a strategist was obvious, and he moved rapidly through northern Kyūshū with a great deal of success, bringing the region under his control by October 1372. This was an impressive achievement considering Prince Kanenaga had been fortifying his position in this region for more than a decade. Kanenaga was not defeated outright however, and went on the defensive, leading to a stalemate that lasted through to 1374, when Kanenaga's general, Kikuchi Takemitsu, died, leaving his military with no strong leader. Sadayo seized the opportunity and planned a final attack.
Sadayo met with three of the most powerful families on Kyūshū to gain their support in the attack, those families being the Shimazu, the Ōtomo and the Shōni. Things seemed to be going well until Sadayo suspected the head of the Shoni family of treachery and had him killed at a drinking party. This outraged the Shimazu clan, who had originally been the ones to convince the Shoni to throw their lot in with Sadayo, and they returned to their province of Satsuma to raise a force against Sadayo. This gave Prince Kanenaga time to regroup, and he forced Sadayo back North, prompting Sadayo to request assistance from the Bakufu.
Help never arrived, forcing Sadayo to take matters into his own hands, and he continued to push the loyalists forces until their resistance ended with Prince Kanenaga's death in 1383. The death of Shimazu clan chieftain Ujihasa in 1385 also helped ease tensions between Sadayo and the Shimazu for a time.
In 1395 both the Ōuchi and Otomo families conspired against Sadayo, informing the Bakufu that he was plotting against the Shogun, in a move that was likely an attempt to restore the post of constable to the family that had held it prior to Sadayo, the Shibukawa family. Sadayo was relieved of his post and returned to the capital. Sadayo had, in addition, acted fairly independently in his negotiations with the Shimazu, the Otomo and the Shoni, and also in negotiations with Korea; this recall was prompted by all three causes being used against him by his enemies in the Shogun's court.
Later years (1395-1420)
In 1400 Sadayo was once again questioned by the Bakufu, this time in relation to the Imagawa's province of Tōtōmi's failure to respond to a levy issued by the Bakufu- a negligence interpretable as treason and rebellion. This charge saw Sadayo stripped of his post as constable of Suruga and Tōtōmi provinces, and gave him reason to believe he might be assassinated. With this in mind he fled the capital for a time, though was later pardoned and returned to the capital, spending the rest of his days pursuing religious devotions and poetry until his death in 1420.
Sadayo began composing poetry from an early age: by the age 20, he had a poem included in an imperial anthology (the Fūga Wakashū or "Collection of Elegance"; Earl Miner gives the specific entry as XV: 1473). His teacher was Reizei no Tamehide (d. 1372). His poems were displayed to more effect in his fairly popular and influential travel diary, Michiyukiburi ("Travellings"). It was this travel diary that in large part won Sadayo a place as a respected critic of poetry: he felt that poetry should be a direct expression of personal experience, a fact that can be seen from his own poems.
Even though Sadayo is better known for his criticism of the more conservative poetry styles, the Nijo school in particular, and his tutoring of Shotetsu (1381–1459), who would become one of the finest waka poets of the fifteenth century, than he is for his own output, it nonetheless provides a glimpse into the mind of this medieval scholar and his travels.
Sadayo was active in the poetic disputes of that day, scoring a signal victory over the Nijō adherents close to the Ashikaga Shogunate at the time with 6 polemical treatises on poetry he wrote between 1403 and 1412, defending the Reizei's poetic doctrine and their cause (despite Ryōshun's renga poetry's debt to Nijō Yoshimoto's (1320–1388) examples and rules of composition). Ryōshun used a number of quotations to bolster his case, including notably a quote of Fujiwara no Teika's, which was that all of the "ten styles" (Teika had defined ten orthodox poetic styles, such as yoen, a style concerned with "ethereal beauty", yugen, the demon-quelling style, or the one the Nijo championed to the exclusion of the other 9, ushin) were licit for poetic use and experimentation, and not merely the Nijō's ushin. With the aid Ryōshun afforded him, Fujiwara no Tanemasa's politicking eventually succeeded in converting the Shogun, ending the matter- until the rival Asukai poetic clan revived the dispute, that is.
- A History of Japan, 1334-1615, by George Sansom, Stanford University Press, reprinted 1991.
- ^ Waiting for the Wind: Thirty-Six Poets of Japan's Late Medieval Age, translated by Steven D. Carter, Columbia University Press, 1989.
- The Imagawa Letter: A Muromachi Warrior's Code of Conduct Which Became a Tokugawa Schoolbook, translated by Carl Steenstrup in Monumenta Nipponica 28:3, 1973.
- Unforgotten dreams: poems by the Zen monk Shōtetsu, 1997. Steven D. Carter, Columbia University Press. ISBN 0-231-10576-2
- An Introduction to Japanese Court Poetry, by Earl Miner. 1968, Stanford University Press, LC 68-17138
- ^ "From Feudal Chieftain to Secular Monarch. The Development of Shogunal Power in Early Muromachi Japan", by Kenneth A. Grossberg. Monumenta Nipponica, Vol. 31, No. 1. (Spring, 1976), pp. 29–49
- "The Imagawa Letter: A Muromachi Warrior's Code of Conduct Which Became a Tokugawa Schoolbook", by Carl Steenstrup. Monumenta Nipponica, Vol. 28, No. 3. (Autumn, 1973), pp. 295–316.