Unless done in self-defense, the UN Charter forbids all use of force taken without the approval of the Security Council. But the meaning of this exception is disputed. Most countries read self-defense as lawful only in response to an attack, but some countries argue it can be lawful against imminent attacks. Very few – with the U.S. being an exception – argue it may be used even against anticipated attacks.

The consequence is that some of Tillerson’s “military options” would be considered lawful by the international community (e.g intercepting missiles in flight), some would be contested