Fourth Geneva Convention
Article 33. No persons may be punished for an offense he or she has not personally committed. Collective penalties and likewise all measures of intimidation or of terrorism are prohibited. Pillage is prohibited. Reprisals against persons and their property are prohibited.
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"Pillage is prohibited."
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And what exactly is it that the USA is doing in Syria at the moment? That's right, pillaging Syria's oil. The USA is, and has been breaking international laws with impunity for a long time.

The Cancer of War: U.S. Admits to Using Radioactive Munitions in Syria
[theantimedia.org]