Well duh, we created Pokemon,and therefore we would win. Just kidding.
Before I start my big post, Let me explain something: As this takes takes place 9 years in the future,we can never really know how it will turn out.
Ok here we go. The humans in the Human Team in Sinnoh are planning how to attack the Pokemon back, When a boy dressed in red enters the room.With him is the legendary Pokemon Trainer, Red, From the Kanto Region. Red explains that the boy with him is a Pokemon Ranger, and that he is the ranger that saved the Oblivia region from it's crisis.The ranger explains that he can recover blueprints of devices such as the Ranger Styler and Steelhead Armor.The Human Organisation agrees,and now Red Speaks again and says that the other regions haven't been attacked yet,and that everyone has their Pokemon in those regions.He explains that they could treat the Pokemon their with extra care and take plainfulls of Pokemon to other Regions and show how happy the other Pokemon are.If Celebi were to travel back in time and kill the people, It would simply create Another time line where there would be no war, and the other timelines would be unaltered. Also, since it's 2020, we may have colonized another planet, and we could set up opertions on that said planet. I doubt Arceus could destroy another planet from Earth.
Okay that's all I got. Easily it can be solved peacefully. There doesn't have to be a war.