Full Idea
A political leader cannot have greater authority over foreigners than over his own subjects; but he may not draw the sword against his own subjects unless they have done wrong; therefore he can only do so against foreigners in the same circumstances.
Gist of Idea
Leaders can only attack foreigners who have done wrong (as with their own subjects)
Source
Francis de Vitoria (On the Law of War [1525], p.303), quoted by Jeff McMahan - Killing in War 1.4
Book Reference
McMahan,Jeff: 'Killing in War' [OUP 2009], p.33
A Reaction
The question would then be whether they have 'done some wrong' against this leader, or against some other people (such as their fellow citizens). That would be the 'intervention' justification. Are they engaged in the wrong, or responsible for it?