No politician or nation, no matter how belligerent, considers itself the attacker—not the most authoritarian dictator any more than the most democratically elected leader.
One simple statement, used by nearly every one, summarizes the trick:
We will not attack first, but if attacked, we will defend ourselves.
Every leader says it their own way.
Once the population believes it, they can feel justified in attacking, feeling and claiming innocence. Neither population has to believe the other, only their own leader. All both leaders have to do is point out any infraction from the other side, no matter how small, to justify escalating. Once the escalations begin, to the extent each population accepted the original belief, both can keep escalating as much as they want.
Populations over and over again fall for the same logic as a sibling saying, “But he started it first!”