Is there a theory about what typically happens to some markets when a country goes to war? If yes, should a passive portfolio be rebalanced accordingly? For example industrial metals.
At a risk of stating the obvious: a passive portfolio doesn't try to speculate on such matters.
Normally, in a war everybody suffers and the entire economy goes down. Military contractors do better than average, but the average sucks. The way to take advantage of knowing a war is coming is to leave as soon as possible.
There are strategic materials that can become valuable in a war, but such investments are generally very specialized and not something an ordinary investor would be in a position to exploit.
The most profitable businesses in war are food, oil, and ammunition.