Here's my "Great War" idea;
It was “The Great War”, the war that nearly ended the civilizations within the Pokemon Universe. Every region was involved, and every region was affected. Lives were lost, cities were ruined and the species known as Pokemon were almost extinct. But just when everyone thought all was lost, the final remaining Pokemon managed to bring peace to the human forces from each region.
After the war, the humans found themselves without anything to go back to. The war they had spent years one had ended in, what seemed to be, the end of civilization. But the Pokemon decided to help the humans, even though they did not deserve it at all. After all, the humans nearly ended life as we knew it. They had lost more then they had ever dreamed of, but they did recover.
After a seven year war, it took Pokemon and humans nearly 20 years in order to reconstruct what was lost from the events. Now that everything had returned to normal, Pokemon journeys had become much less popular after the war. Adolescents wanted some kind of adventure for themselves like there was back before the war.