I am a big fan of Saito Dosan since he was one of the very few who understood Nobunaga. It was a blessing that I was able to have the chance to live in Gifu and study Nobunaga and Dosan.
Saito Dosan was born in 1494 and he was first known as Minemaru. Then just before he became a teenager, Dosan entered Myokakuji Temple in Kyoto and was known as Horenbo.
He later became an Oil dealer and more important, a Sengoku warlord. Dosan's nickname was "The Viper of Mino." He made Mino into a prosperous domain due to free market principles and was able to defeat Oda Nobuhide in the 1540s. Eventually, Nobuhide and Dosan made peace and Nobunaga would have the Viper's daughter Nohime (1535-1612) as his wife.
So where did Dosan's decline began? The starting point is the meeting between Dosan and Nobunaga at Shotokuji Temple in 1553. Everyone thought his son-in-law was a fool. However, the meeting between the two changed everything. Dosan right away knew Nobunaga was no fool and the Oda/Saito alliance was good enough to continue. One reason why Dosan knew his son-in-law was no fool since both were self-made men. Dosan saw a little bit of himself when he met Nobunaga. As for Nobunaga, Dosan would be his last parent as his father Nobuhide died in 1551 and his adviser Hirate Masahide took his own life in 1553.
Dosan did provide aid to his son-in-law in 1554 at the Battle of Muraki against the Imagawa. It was a victory for Nobunaga. As for Dosan's son, Yoshitatsu, he was not a happy man. all the love went to Nobunaga. In Yoshitatsu's mind, Nobunaga was still the fool from Owari. This was a drastic mistake on Yoshitatsu part for underestimating Nobunaga's true abilities.
Dosan then gave Yoshitatsu Inabayama Castle (Gifu) and was the new heir to the Saito throne. As for Dosan, he moved to his retirement castle, Sagiyama. In a way, Dosan sealed his own death warrant. Yoshitatsu now had everything including the Mino Big Three (Inaba Ittetsu, Ando Morinari, and Ujie Bokuzen). The situation turned to worse when Yoshitatsu tries to find out who is his real father.
Dosan had took in a wife or concubine by the name of Miyoshino (Inaba Ittetsu's sister). Miyoshino at the time was with Toki Yoshinari (1501-82). Dosan later banished Yoshinari from Mino. Miyoshino then had a child, and it was Yoshitatsu. The debate now, was Miyoshino pregnant when Dosan took her in? I highly doubt it. Yoshitatsu then believes his true father was Toki Yoshinari. Blood starts to boil as the two cannot get along and it nothing but bitterness.
Dosan then married Nohime's mother Omi no Kata (1513?-51) around 1533. Omi no Kata was the daughter of Akechi Suruga no Kami Mitsutsugu and gave she birth to Nohime in 1535. It was also possible she was the mother of Dosan's son's Magoshiro and Kiheiji.
It got even uglier when Yoshitatsu killed Dosan's son's Magoshiro and Kiheiji in 1555. It was now all out war and Dosan knew his time was near. Dosan was outnumbered and Mino Big Three switched over to Yoshitatsu. Mori Yoshinari (Mori Ranmaru's father) would leave the Saito and joined Nobunaga. Just before the Battle of Nagaragawa in 1556, Dosan wrote his will and gave the Mino deed to Nobunaga. Nobunaga tried to lead his army to help his father-in-law, but it was too late. Dosan was killed in battle and Yoshitatsu was now the supreme ruler of Mino.
Dosan's grave (Gifu City).
Nobunaga never had the chance to provide aid for Dosan. Two reasons why. First, Kiyosu was too far away from Mino. Nobunaga realized this in the earlier 1560s and he moved his headquarters from Kiyosu to Komaki in 1563. Second, Owari was not yet fully unified. A major problem since he still had enemies in his domain. If Owari was unified, Dosan still had complete control of Mino, and the Mino Big Three stays with the Viper. Yoshitatsu would never revolted against his father.
Nobunaga scored a major coup when he defeated Imagawa Yoshimoto in 1560. Saito Yoshitatsu knew he was next. The chickens came to roost. He probably was thinking Dosan was right from the start that Nobunaga was no fool. Yoshitatsu passed away in 1561 and Nobunaga quickly attacked Mino and defeated the Saito at the Battle of Moribe (1561). Mino would be a tough nut to crack even with a young Saito Tatsuoki as the heir. Nobunaga would soon chip away the Saito and convince the Mino Big Three (Inaba, Ando, and Ujie) to switch to the Oda. With the Mino deed as his backing, Nobunaga eventually conqueror Mino in 1567 and changed Inabayama to Gifu.
Saito Dosan was a classic example who used gekokujo to the fullest extent. He knew how to win on and off the battlefield using politics, economics, and of course war. When the perfect opportunity came, the Viper of Mino took full advantage.
Tenka no tame!