Chiefly by a code of death-before-dishonor (and death-after-dishonor) which makes sense for a warring country to precommit to. Though it doesn't seem there was much conscious reasoning that went into the code's establishment, just an evolutionary optimization on codes of honor among rival daimyo, which resulted in the entire country having the values of the victorious shoguns instilled.

I'm no history expert, but I remember hearing something about cutting off a finger and promising to kill anyone that shows up missing that finger.