Please go to province-defence for information on Province Defence.

The Province Defense is not immensely respected by your more professional armies, in case them being treated as a separate army under your flag wasn't a clear-enough tell. Their squads are roughly the same as the different rows in the Defense menu, and your stray troops simply left in the province are not integrated into them. Despite being a separate army, they HP-rout with your professional army, and technically can be the ones to cause an HP-rout; however, their HP is only 25% as relevant as that of your professional troops.

You can't script or reorganize your Province Defense, but it replenishes each turn for free. The only way to totally wipe a Province Defense level is by taking the province for yourself, or by killing the entire local population with Lost Land or other magic.

Ultimately, some nations have much better province defenses than others, especially for the cash investment required; and on the other hand, some nations are far better at trivializing Province Defense than others, whether through immensely powerful units or Commander-sniping shenanigans.