Combat immediately lost, nomatter how weak opponent...

[taltamir]

did you LOSE some leadership?
If your leadership is less then your troops require, they will be "out of control", depending on the amount and the difficulty that might mean they are berserk attacking enemies automatically, or they might attack friendlies as well.
I would imagine that if you start combat with all of them out of control you will lose combat instantly. Go to here info, if you see a red fire icon on a creature it means it is out of control, you will also see it written down in its description, and if you go to dismiss it, it will automatically suggest as many needed to get creature back under control, instead of the whole stack like it normally auto suggests.