A relatively current comment of the international law of occupation is found stated at : ICRC.org
Also, see Occupied.tc BLOG
In summary it may be understood that :
"Article 42 of the 1907 Hague Regulations (HR) states that a " 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. "
According to their common Article 2, the four Geneva Conventions of 1949 apply to any territory occupied during international hostilities. They also apply in situations where the occupation of state territory meets with no armed resistance.
The legality of any particular occupation is regulated by the UN Charter and the law known as jus ad bellum. Once a situation exists which factually amounts to an occupation the law of occupation applies – whether or not the occupation is considered lawful.
Therefore, for the applicability of the law of occupation, it makes no difference whether an occupation has received Security Council approval, what its aim is, or indeed whether it is called an “invasion”, “liberation”, “administration” or “occupation”. As the law of occupation is primarily motivated by humanitarian considerations, it is solely the facts on the ground that determine its application."
Certainly, an important site to visit is the International Law of War site : the reources are military lawyers, academics; and other sources such as judiciary and government officials.
Reference Index
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Carnage and Culture : Professor Hanson,
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Treaties Signed While Under Occupation May Not Be Valid By International Law