Im assuming the game tells you when your going to war. I make saves pretty often, but I just want heads up its coming so that i can then go get the pin.
the game is realy explicit, it's the big event in the end of the chapter, you must make a decision how starting the invasion, and this is the last round of investment before the war.
you can open the ruins after the war, but before the end of chapter four (there is a last investment round after the erosia war), you can grab the pin too, the pin give a little bonus during a specific event in the war, because this the
best moment is just before the war, but immediately after is a legit choice.