I send my army out to scout.
Enemy army gets around my army somehow and captures one of my cities without a fight.
I move my army back, bringing in a second army from a second city. The two armies attack my captured city at the same time.
It's a knock-down drag-out bloodbath. I outnumber the enemy nearly 2-1, but their army is almost entirely composed of powerful Tier 3 units, plus the enemy leader. They also have those nice walls I spent time and money building. The enemy leader summoned stupidly powerful fireballs every turn, dealing massive amounts of unavoidable damage. By the end of the battle, I had only two units left: a near-death low-tier magic initiate and the hero who lead the two armies. The hero actually fell during the battle, but he had an ability that would bring him back to life if his army won anyway.
So I have my city back. I immediately order the training of additional units to replace my losses.
And then the city is attacked. By another army of Tier 3 units. These ones can fly, rendering my walls useless. They can also use some sort of curse ability that hits my hero unit from a ridiculous distance and through the walls.
I struggle valiantly, but my pitiful "army" falls without slaying a single enemy unit. The city is lost (again) and the hero dies for good this time.
Oh right. The hero was mission-critical. Game over, time to restart the campaign map.