Saitō Dōsan (斎藤 道三, 1494–May 28, 1556) was a daimyo who dramatically rose and also fell from power in Sengoku period Japan. He was also known as the Serpent of Mino (美濃の蝮 Mino no Mamushi) for his ruthless tactics.
Originally a wealthy merchant from Yamashiro Province (modern-day Kyoto Prefecture), he entered the service of Nagai Nagahiro of Mino Province (southern half of modern-day Gifu Prefecture), assuming the name Nishimura Kankurô.
He used his power and influence to become a retainer of the daimyo of Mino, Toki Yorinari.
Dōsan contributed to general instability within Mino Province, so Yorinari gave him his concubine in the hopes that this would appease him in 1526.
He later married Ômi no kata, a daughter of Akechi Suruga no kami Mitsutsugu.
He eventually succeeded in becoming the magistrate of Mino Province and settled in Inabayama Castle. Using his power and wealth, he drove Toki Yorinari out of Mino Province in a coup d'état in 1542, and claimed the region as his own, becoming a daimyo in his own right. Afterwards, Toki Yorinari allied with Oda Nobuhide of Owari Province, which was on the southern border of Mino Province, but their defeat at the Battle of Kanōguchi, in 1547, solidified Dōsan's domination of Mino and also made him known throughout Japan. Oda Nobuhide made peace and arranged a political marriage in 1549, between his son, Oda Nobunaga, and Dōsan's daughter, Nōhime, to end all hostilities.
Ironically, Saitō Dōsan fell in his own son Saitō Yoshitatsu's coup d'état in 1556.
Around 1555, rumors began to circulate that Saitō Yoshitatsu was not in fact Dōsan's son; it was said that he was Yorinari's. It does not appear that Yoshitatsu had been aware of that possibility himself until he heard the rumors.
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