Full Idea
It is possible for a defensive war to be unjust, when the defensive war to which it is a response is a just war.
Gist of Idea
A defensive war is unjust, if it is responding to a just war
Source
Jeff McMahan (Killing in War [2009], 3.3.3)
Book Reference
McMahan,Jeff: 'Killing in War' [OUP 2009], p.147
A Reaction
An example might be a state resisting an intervention from outside, when the state is in the process of exterminating some unwanted minority. Or perhaps the invaders are crossing the state's territory to achieve some admirable end.