In the Vrynenwood, the leaves are not what they seem.
When an opposing army brings in the big weapons, your kingdom has no choice but to ante up or fold - and a trained Battle Mage from the Guild of Arcane Combatants if nothing if not a big weapon. But what happens your generals and their generals have a common goal?
Real World: some Indians in the Amazon treat their eyes with a traditional potion applied with palm leaves. Brutally painful, the drug alters vision, giving the jungle's dense green walls greater texture and dimension. You could adapt this to desert or swamps, or other hard to navigate regions.