Full Idea
If I wage war at the commandment of my Prince, conceiving the war to be justly undertaken, I do not therefore do unjustly, but rather if I refuse to do it, arrogating to myself the knowledge of what is just and unjust, which pertains only to my Prince.
Gist of Idea
I act justly if I follow my Prince in an apparently unjust war, and refusing to fight would be injustice
Source
Thomas Hobbes (De Cive [1642], 12.II), quoted by Jeff McMahan - Killing in War 2.6
Book Reference
McMahan,Jeff: 'Killing in War' [OUP 2009], p.85
A Reaction
Hobbes early says that Princes make things just by commanding them. This presumably assumes divine authority in the Prince. This is, of course, ancient pernicious nonsense.