Why is propaganda a necessity of war?

Propoganda, in general, is a method of portraying information so that your view or actions look noble and right as opposed to your enemy who is portrayed as cowardly and dastardly. During war propoganda is used to attack the enemy over the airwaves and in print while attempting to keep the homefront calm by portraying the cause as just and noble.

Explanation:

china unilaterally claims the entire south china sea as sovereign territory, then builds artificial islands to bolster its claim. it suddenly activates an air defense identification zone over the east china sea, and threatens to down any aircraft that does not report its position. it builds roads into indian territory, then redraws the maps to show that it is actually chinese territory. the people's republic under president xi jinping is quickly becoming the bully of asia.