Tokugawa Ieyasu had won the Battle of Sekigahara in 1600 and set up organising Japan under his clan’s domination.  However, safely ensconced in Osaka Castle was Hideyori, son of Hideyoshi; the last of the Toyotomi.  Tokugawa Ieyasu knew that he would never be safe until the last of the Toyotomi claims to control of Japan were snuffed out.