Threats of force are a violation of international law
'Friendly Relations Declaration'
Declaration on Principles of International Law concerning Friendly Relations and Co-operation among States in accordance with the Charter of the United Nations, 24 October 1970.
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Every State has the duty to refrain in its international relations from the threat or use of force against the territorial integrity or political independence of any State, or any other manner inconsistent with the purposes of the United Nations. Such threats or use of force constitutes a violation of international law and the Charter of the United Nations and shall never be employed as a means of settling international issues.
A war of aggression constitutes a crime against the peace, for which there is responsibility under international law.
In accordance with the purposes and principles of the United Nations, States have the duty to refrain from propaganda for wars of aggression.
Every State has the duty to refrain from the threat or use of force to violate the existing international boundaries of another State or as means of solving international disputes, including territorial disputes and problems concerning frontiers of States.
(bold emphasis mine)
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