According to a developer response here, squads roll (0-ATK/NumTroops) against (0-DEF/NumTroops) in combat.
The current way of showing a sum of ATK and DEF on the unit cards is very misleading. Those numbers are never used for anything and require knowledge not available in the game to understand.
Let's take an example of an 8 man squad with 64 ATK and 64 DEF fighting against a giant with 50 ATK 40 DEF. It looks like the squad is stronger, right? Well, they are probably going to get clobbered to death by the giant since we actually have 8 rolls of 0-8 versus rolls of 0-40. The defense is being rolled in full strength against each hit. And hitting the other way around we have 0-50 versus 0-8.
The giant is clearly the stronger in combat, but there's absolutely no information to support this inside the game.
Please let squads show the ATK and DEF of individuals, not a sum! This never caused misunderstandings with MoM...