You always justify it with something the people you need to support you deeply believe.

For instance, in Brazil, we went through a coup and elected a fascist because most people really believe the previous two governments were corrupt and would implement a communist dictatorship given time.

The fact they are not measurably more or less corrupt than their predecessors, that they formed a coalition with Center-right parties in order to govern, and that their platform is timidly social democratic were, and continue to be, largely ignored.

To the point that now, after more than 600 thousand avoidable deaths, unprecedented unemployment, countless corruption scandals directly connected to him, and the worst economic crisis since the 90’s, only 58% support his impeachment.

But you have no war yet.

If you really want to know how to manufacture consent for war, you should study Goebbels works, or Russian propaganda (Putin likes Goebbels).

You sponsor coups so you don't need a war. Much cheaper, and also saves a lot of face.