I am about to start a new campaign playing as Tokugawa and was intrigued to discover that in Shogun 2 this clan begins it's campaign as a vassal of the Imagawa Clan to the south, and thus unable to declare war on any clan which it's master are not already at war with. I was curious as to how and why the Tokugawa clan had become vassals of the Imagawa. According to the briefing this came about as a result of the murder of the clans Diamyo Matsudaira Kiyoyasu by Abe Sadayoshi one of his retainers .
Matsudaira Kiyoyasu
This led to a civil war within the clan though I'm still not clear how this eventually led to the Tokugawa Clan becoming vassals of the Imagawa, but we are told in the briefing that it did. The result of this left the Tokugawa clan facing constant threats and raids from the Oda clan to the north and being forced to shed it's blood to help protect the Imagawa from the ambitions of Lord Oda.
What I did discover was this wonderful Japanese film about the trials and tribulations of the Tokugawa Clan which sets the scene quite nicely for the opening of the campaign.