You can only select to "prepare for a possible war" yet, which is all you need for other events like with Kurohime. The actual wars will happen after the current end of the main story, in the "time skip" during which the time in the game is advanced in multiple phases. A test version of the pre-war and early war phase is available via the sparkling book in your bedroom, but it has no effect on anything else in the game right now.
