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 geo-political entity (either adjacent or non-contiguous). After World War I Alsace and Lorraine were a formal detachment from Germany. More often detachment occurs as a process within a country, for example the creation of the Federal District of Columbia resulted from a detachment of territory from the State of Maryland. The removal of territory from a city or special district is a detachment. Within a country detachment is governed by the laws of the supervening entity. Detachment can be considered the opposite or reverse of annexation.
Detachment (territory) Wikipedia
The formal detachment of Egypt from the Turkish Empire was a condition for British investment in the Suez Canal.
Following World War I, a number of colonial territories and border territories were detached from Germany, as well as portions of the Austrian-Hungarian Empire and Ottoman Empire being detached. Some of these detachments were incorporated directly into new countries, such as Yugoslavia, or annexed by existing countries such as Northern Schleswig into Denmark. But some, particularly in the Middle East and those of the German colonies, were placed under the "protection" of one or another of the Allied countries who won the war, including Germany's concessions in China, Kiautschou and Chefoo. From a rule of law standpoint these protectorates were not war booty, but "mandates" from a legally constituted international body, so detachment occurred without annexation.