Secession

Formal withdrawal of a group from a political entity

Secession is the unilateral withdrawal of an integral part of a state's territory. Secession generally aims to establish a separate political entity, typically a sovereign state, although it may instead seek accession to another existing state. Secession often begins with an act such as a declaration of independence, and may be pursued peacefully or through armed conflict.

From Wikipedia, under CC BY-SA. More on occurri.