In *The Battle of Five Armies*, Thorin's cousin Dáin leads a charge of dwarfs against some orcs to defend the lonely mountain. Why does he leave half of his host behind just watching the fight? (I mean, there is seriously no reason to leave them there since the elves and dwarfs put aside their original conflict.)

For context, here’s a picture ([imgur source](https://imgur.com/N6uffd8)):

![enter image description here](https://i.stack.imgur.com/R15WT.jpg)