Full Idea
It is said that combatants are compelled to fight; they have no choice. But duress is not a justification; nor does it ground a permission - not even a subjective permission. It is, instead, an excusing condition.
Gist of Idea
Fighting unjustly under duress does not justify it, or permit it, but it may excuse it
Source
Jeff McMahan (Killing in War [2009], 3.1.2)
Book Reference
McMahan,Jeff: 'Killing in War' [OUP 2009], p.113
A Reaction
The 'subjective' permission is believing you are just, even if you aren't. A nice, accurate and true distinction made by McMahan, I think. It is roughly our postwar attitude to the Nazi army.