Not a historian, but I have an ancient history degree, so i'll avoid answering any specifics for the time period you're asking about.
I think your question puts the cart before the horse: justification doesn't allow action, the action generates justifications.
It's not the strengths of the justifications or explanations that allow powerful states to exert their power. Power - violence, knowledge, organisation etc. - allows states to threaten or overcome those who would oppose them. The only states with the sustained capacity to oppose any one state's goals were other european powers, hence dividing it up between them.
Going back in time a bit, David Graeber in Debt talks about how actually very few people really tried to justify slavery. Some, sure. Industrial slaveholders. But more common was "what can we do about it? It's out of our control". How do people justify sweatshops, iphone factory conditions today? For the most part they don't. Some do and it's usually cringe: flimsy, transparently cruel, self-serving.
Mostly we all know it's unjustifiable and accept it as the way of the world.