Assaults are usually very decisive — one side or the other quickly gains the upper hand and forces back their foe. Numbers and leadership can keep a side in the fight but the casualties each side inflicts is the most telling factor.
To decide who has won the combat, total up the number of wounds inflicted by each side. The side that causes the most is the winner, the other side is the loser and may be forced back if they fail a Morale check. Note that wounds which have been negated by armour saves do not count, nor do wounds in excess of a model's Wounds characteristic — only wounds actually inflicted.
If both sides score the same number of wounds the combat is drawn and continues next turn.
Wipe Out!
If one side destroys the enemy it wins automatically – even if it sustained more casualties. They may consolidate as normal (see Losers Break Off and Winners Consolidate). In addition the destruction of the unit will unnerve nearby friends if they are not already Engaged in close combat. Take a morale check for every unengaged friendly unit within 6" of the victorious unit before it consolidates. If the check is failed, the unit in question will fall back as described in Fall Back!.
Designers Note: The rule above is borrowed from Warhammer Fantasy Battle, which quite sensibly supposes that troops seeing their fellows wiped out to a man will start giving serious thought to hoofing it before they're next. It means that tough assault units can break an enemy gun line, throwing the foe into confusion as they try to pull back. This helps balance off some of the risks intrinsic in braving enemy fire to assault in the first place, and the vulnerability of assaulters to getting blasted with supporting fire the moment they emerge from a close combat.