I've been reading about the Second Punic War lately, and I keep coming across instances of armies lining up to fight a pitched battle, and then one or both declining to fight.
I seem to recall more than a few occasions of this behavior in Italy, and Scipio's most famous battle in Spain was won because they spent a few days lining up and not fighting enough that Scipio could trick Mago into using his same deployment. Livy even describes Hannibal as "refraining from battle" when the Carpetani are descending on him north of the Tagus.
What am I missing here? I can understand the lack of a battle when both armies don't want to fight, but how is it possible that one army could decline to fight, and the battle wouldn't happen? How could Hannibal refrain from battle and just encamp on the river bank? Scipio was declining battle so he could bait Mago presumably because Mago held the upper hand. Why would any general who had the upper hand and seeing that his enemy didn't want to fight refrain from ordering an attack (aside from obvious reasons like the weather or unfavorable terrain that negated his advantage)?
Usually armies would set their camps on good defensive positions - ideally on a hill facing the enemy. If a general wanted to deploy his army but didn't feel confident in actually risking battle then he would simply stay on the high ground near his camp. Any enemy attacking him would have to move uphill - tiring his men out, making thrown missiles a little less effective, and giving a momentum and psychological advantage to the more defensive general if he decided to switch over to the attack. There was also the added benefit that (for the Romans) the defended camp was nearby to seek refuge within and reorganize for a counterattack if things went pear-shaped. The opponent would probably be at a disadvantage in that they'd have broken formation trying to chase down a fleeing army and very vulnerable to a counterattack.
So why even bother deploying? A good reason is to keep the men's morale up. Staying in camp was an admission that your army couldn't win - which probably didn't do wonders for the soldier's morale. It's a way of showing your willingness to fight - to a point. There's one anecdote where a Roman general explained to his men his plan to stay inside the camp for several days when confronted by a rather fleet-footed enemy to feign fear. Their opponent became contemptuous and moved right up to the camp and were caught flat-footed when the 'scared' Romans flooded out and routed them.
In ancient battles, they generally occurred when both sides wanted to fight and would move down from more advantageous ground to a position where each side felt wouldn't be at a significant disadvantage.