"Persuade thyself that imperfection and inconvenience are the natural lot of mortals, and there will be no room for discontent, neither for despair." - Tokugawa Ieyasu, Unifier of Japan, Founder of the Tokugawa Shogunate
I've studied the Sengoku Era of Japanese history a lot. There are so many lessons in it - about courage, about restraint, about going too far, about not going far enough... about honor, about idealism, about pragmatism... about trust and distrust, love and hate, loyalty and betrayal...
It's rare that an era of history has so many unique and varied lessons to teach. The only thing that comes close, in my mind, is the Italian Renaissance. Certainly, there's been eras with a great many lessons to learn from them, but not so many with such a wildly diverse range of views. Sengoku was the crossroads that created Japan. The victors of the era were those who could appeal to tradition while using the most modern advantages - tapping into the samurai culture and spirit, while simultaneously beginning to employ firearms and other newly emergent technologies in war.
Out of Sengoku came Japan's "Three Great Unifiers" - Oda Nobunaga, Toyotomi Hideyoshi, and Tokugawa Ieyasu. It's a long story and you can read the history article I wrote on Sengoku that I linked if you're curious to see the whole thing.
But the basic idea is, Nobunaga and Hideyoshi were both probably more remarkable and more brilliant men than Ieyasu. But in the end, Ieyasu won and his family and administration ruled Japan for the next 250 years.