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Tree at Tsurugaoka Hachiman Shrine. Sanetomo was assassinated by his nephew Kugyō Minamoto hiding behind this tree.

Minamoto no Sanetomo (源 実朝, September 17, 1192 – February 13, 1219, r. 1203–1219) was the third shogun of the Kamakura shogunate and the last head of the Minamoto clan of Japan. Sanetomo was the second son of the founder of the Kamakura shogunate Minamoto no Yoritomo, his mother was Hōjō Masako, and his older brother was the second Kamakura shogun Minamoto no Yoriie.

After the death of his father Yoritomo in 1199, Sanetomo's grandfather Hōjō Tokimasa usurped all political and military power of the shogunate, relegating the position and title of Seii Taishogun, or shogun, to a mere figurehead. Through hereditary succession, Sanetomo's older brother Yoriie became Seii Taishogun in 1202, to only be stripped of the title a year later and put under house arrest for plotting against the Hōjō clan. Shortly thereafter in 1203, Sanetomo became head of the Minamoto clan and was appointed Seii Taishogun. In the next year, 1204, Yoriie was assassinated by the Hōjō. Sanetomo was never anything more than a puppet for his mother Hōjō Masako, who used him as a pawn in her war with Tokimasa- Tokimasa would try to depose his grandson a number of times, beginning in 1205, causing Sanetomo to fear for his life the rest of his days.

Sanetomo, understanding his own powerlessness in comparison to the Hōjō and not wanting to meet the same fate as his brother, put all of his time and energy into writing poetry and gaining posts within the powerless but honorary imperial court. Sanetomo was a talented poet, writing over 700 poems between the ages of 17 and 22 while being tutored by Fujiwara no Teika, even having one of his tanka included in the anthology Ogura Hyakunin Isshu (100 Poems by 100 Poets), a noted collection of Japanese poems of the Heian and early Kamakura periods. Sanetomo also achieved the third highest post of the imperial court, Minister of the Right (Udaijin), a type of Vice-Minister, in 1218. Eventually, he lapsed into inactivity and despair, plagued by fear of assassination and tormented by his chronic alcoholism (an addiction which Priest Eisai once tried to break by replacing alcohol with tea).

Sanetomo was assassinated on New Year's Day 1219 by his nephew Kugyō Minamoto (1200-1219) - son of the second Kamakura shogun Yoriie, at the steps of Tsurugaoka Hachiman Shrine in Kamakura, where his nephew served as the chief priest. With his assassination, the Seiwa Genji line of Minamoto came to an end.

Minamoto no Sanetomo was succeeded by Kujō Yoritsune as fourth shogun of the Kamakura shogunate.