Dappan (Leaving the Domain) (脱藩)
"Dappan" was the act of a samurai leaving his master's domain to become a Ronin (masterless samurai).
Summary
During the Sengoku Period of Japan it was a frequent occurrence for samurai's to part from their masters, but from the Edo Period it became an unforgiveable crime for a vassal to abandon his lord and many times pursuers were sent after them. This was because, through the Dappan samurai, the Domain's military secrecy and family troubles could be revealed in public and frequently caused fatal 'Kaieki' sanctions (sudden dismissal and deprivation of position, privileges and properties) for the domain (and/or the lord of the domain).
From the mid-Edo Period to the Taihei Period, military secrecy was no longer needed and ongoing financial difficulty made it possible for vassals to leave, unless they served in key positions. By then it was also a requirement to go through legal procedures in order to leave, and those who left without it were charged with desertion. Offenders had their family name declared extinct, their estates confiscated, and if they were caught, they faced the possibility of execution.
Towards the end of the Edo period, many samurai were dissatisfied with their lack of freedom within their domains and opted to leave their domain and back the Sonno Joi ideal (Imperialism and the expulsion of foreigners). In many cases, the domains overlooked those who left. Well-known "Dappan" samurai include Shoin YOSHIDA, Ryoma SAKAMOTO, Shintaro NAKAOKA and so on.