Declaration on the Inadmissibility of Intervention and Interference in the Internal Affairs of States ~ United Nations #Yemen

(c) The duty of a State to refrain from armed
intervention, subversion, military occupation or any other form of intervention and interference, overt or covert, directed at another State or group of States, or any act of military, political or economic interference in the internal affairs of
another State, including acts of reprisal involving the use of
force;

http://www.un.org/documents/ga/res/36/a36r103.htm