War is not in the game yet. The preparation will be things you can do in the "current time", and the actual war will be added in the time skip events of the main story.
If I understand correctly, that if you finish all quests and story lines, then you will be able to jump in time into the future, where you will meet your children and you will train them to war against Raizel?
It's not in the game yet, but you're mostly correct. At the current end of the main story, there will be a time skip event where you'll see things that happen during the time you prepare for the war against Raziel, including your war with other countries and your children getting born and grown up.
About wars, Theremis constantly said "war would currently cause more troubles than we would gain from it"? Or have possible how do war and not have troubles?)
You can "prepare for war" but not actually start the wars in the "current time". In the time skip phase, it'll be possible to have wars with other countries. A preview for this is available via the sparkling book in your bedroom, but it has no effect on the rest of the game at the moment.
