Detachment (Old French de, from, and [at]tach, joining with a stake) under international law is the formal, permanent separation of and loss of sovereignty over some territory to another geopolitical entity (either adjacent or noncontiguous). Detachment can be considered the opposite or reverse of annexation.
A prominent example of detachment is the official and formal relinquishment of Alsace and Lorraine by Germany, following World War I. More often, however, detachment is a result of the creation of a new, sub-national geographical entity within one country. When detachment occurs within a country, the new entity is usually administered subsequently by a supervening entity, such as a national/federal government. For example, after the United States became independent, it was considered desirable, for various reasons, for the federal capital to be situated beyond the boundaries and jurisdiction of the constituent States. Consequently, in 1790, the States of Maryland and Virginia agreed to permanently detach adjoining areas on their border, to become the District of Columbia (DC), including the site of the future city of Washington DC. The formal removal of a smaller area from a city, town, or incorporated, non-urban district is also considered to be a form of detachment. For example, while the city of Alexandria, Virginia and the neighboring Alexandria County were detached from Virginia, to become a founding parts of the District of Columbia, the residents of Alexandria and Alexandria County (later Arlington County) began to campaign for the area's "retrocession" (or reattachment) to Virginia. This occurred in 1847.