How is it possible to kill units in burning zones?

As far as I understand the rules, I can attack a burning zone but my opponent has no reason to defend with any units because the zone is already burning, Therefore the opponents units are quite save and can procuse resources undisturbed. I find this pretty weird, as it should be possible to fight against those units. Do I get something wrong?

Thanks for your help.

You need cards that allow you to target specific Units (Flames of Tzeentch, Forced March, etc.) to either kill them where they are or move them to another zone. Troll Vomit and Judgment of Verena (+removing the developments in that zone) work. Moving damage from your Units to opponent's Units. Malus Darkblade.

At first, I thought it was strange that a zone was no weaker when burning and almost made that player stronger - apart from being halfway to conquest - with one less zone to protect. However, if a player is putting units into a burning zone that is less they have to protect the others. You will find many more cards doing direct damage in the Battle Packs, including those that force a unit to defend, so you will be able to remove a pesky annoying unit hiding in a burning zone.

Cheers

There's also this neutral 2-cost tactic card from The Path Of Zealot:

ANIMOSITY

Play after a zone is attacked.
Action : Target unit must defend this turn, if able.

Thanks for your replys! So I got the rules right but need to understand the cards better (... and get me those battle packs ... )