yes, the default setting is that you get some 60-100 something enemies counterattack once you've captured drassen mine
the default settings will make it so that you face all of them after each other, given you can only battle 32 at a time (even that'll be changed at some point)
there's a lot of things you can do to avoid this:
STRATEGIC_EVENTS_SEND_TROOPS_TO_DRASSEN can be set to FALSE
then, reinforcements can be disabled. This means, the enemy troop pool from the three surrounding sectors can't hit at the same time --> fight split up in smaller parts
also, the attack only happens once you have the drassen mine captured. So you can, for example, conquer the two other sectors first, then train an awful lot of miltia in the other sectors (veteran miltia traning can be enabled in .INI)
you can also try to fight some of the approaching enemies before they reach drassen or you can conquer a different town first altogether so you can get some money and supplies first. I usually hit the pool of enemies efore they attack too. They gather in 3 sectors, i disable reinforcements and hit some of these sectors first.
But the easiest way is probably to conquer lots of other stuff first, get sniper rifles and grenade launchers and good mercs etc and have some carnage
an other tactic is to retreat an re-attack at night. Last time i tried that tactic, it was rather easy due to AI. But i don't know if everything is still the same, haven't played an unmodified 1.13 in ages