Military occupation occurs when the control and authority over a territory passes to a hostile army

Belligerent

A belligerent is an individual, group, country or other entity which acts in a hostile manner, such as engaging in combat. Belligerent comes from Latin, literally meaning "to wage war"...

. The territory then becomes occupied territory.

Military occupation and the laws of war

From the second half of the 18th century onwards, international law has come to distinguish between the military occupation of a country and territorial acquisition by invasion and annexation, the difference between the two being originally expounded upon by Emerich de Vattel

Emerich de Vattel

Emer de Vattel was a Swiss philosopher, diplomat, and legal expert whose theories laid the foundation of modern international law and political philosophy. He was born in Couvet in Neuchatel, Switzerland in 1714 and died in 1767 of edema...

 in The Law of Nations (1758). The distinction then became clear and has been recognized among the principles of international law since the end of the Napoleonic wars in the 19th century. These customary laws of belligerent occupation which evolved as part of the laws of war

Laws of war

The law of war is a body of law concerning acceptable justifications to engage in war and the limits to acceptable wartime conduct...

 gave some protection to the population under the military occupation of a belligerent power.

The Hague Conventions of 1907

Hague Conventions (1899 and 1907)

The Hague Conventions were two international treaties negotiated at international peace conferences at The Hague in the Netherlands: The First Hague Conference in 1899 and the Second Hague Conference in 1907...

 further clarified and supplemented these customary laws. Specifically "Laws and Customs of War on Land" (Hague IV); October 18, 1907: "Section III Military Authority over the territory of the hostile State." The first two articles of that section state:

Art. 42.

Territory is considered occupied when it is actually placed under the authority of the hostile army.

The occupation extends only to the territory where such authority has been established and can be exercised.

Art. 43.

The authority of the legitimate power having in fact passed into the hands of the occupant, the latter shall take all the measures in his power to restore, and ensure, as far as possible, public order and safety, while respecting, unless absolutely prevented, the laws in force in the country.

In 1949 these laws governing belligerent occupation of an enemy state's territory were further extended by the adoption of the Fourth Geneva Convention

Fourth Geneva Convention

The Geneva Convention relative to the Protection of Civilian Persons in Time of War, commonly referred to as the Fourth Geneva Convention and abbreviated as GCIV, is one of the four treaties of the Geneva Conventions. It was adopted in August 1949, and defines humanitarian protections for civilians...

 (GCIV). Much of GCIV is relevant to protected persons in occupied territories and Section III: Occupied territories is a specific section covering the issue.