Pokemon: What happen? Human: Someone set us up the voltorb. Pokemon: We get signal. Human: What? Pokemon: Main PC Box turn on. Human: It's you! AZ: How are you gentlemon! AZ: All your soul are belong to wepon. AZ: You are on the way to destruction. Human: What you say! AZ: You have no chance to survive make your time. AZ: Ha ha ha ha... Pokemon: Captain! Human: Take off every poke ball! Human: You know what you doing. Human: move poke ball! Human: ...for great justice.
If you die around here, you'll burn so hard there won't even be ashes left. If I want your corpse, I'd better take ya down here!
You mean that war that was related to AZ's back-story?
IIRC, there were some book shelves over Lysandre Labs that gave quite a bit of info on that.
Basically, AZ's kingdom was peaceful and rich, but it was targeted by greedy people that wanted to take it over (one such people being AZ's brother, whom Lysandre is a descendant). And yes, people used Pokemon to wage wars back then.
The rest of the story, AZ tells you while he's imprisoned during Team Flare's arc.
So the reason that war happened was because mankind are dicks (what else is new...).
"Many of you feel bad for this lamp...That is because you're crazy. It has no feelings! And the new one is much better." (IKEA's Lamp commercial)