In D&D 3.5 the Withdraw action is defined as follows:
Withdrawing from melee combat is a full-round action. When you
withdraw, you can move up to double your speed. The square you start
out in is not considered threatened by any opponent you can see, and
therefore visible enemies do not get attacks of opportunity against
you when you move from that square.
What I don't get is that an Attack of Opportunity is basically a free attack by your enemy because for one reason or another you let your guard down by casting a spell, or firing a ranged weapon, etc., etc. Basically you're not fully engaged in defending yourself.
How does fleeing combat not cause an Attack of Opportunity since you're basically fleeing combat?
Best Answer
It's a full round action.
Think of it like this: You are using the move action to move away, while using your standard action to pay attention to your surroundings, to make sure that no one can take advantage of the situation.
You do not just run away and flee, you do not let your guard down. Instead, you retreat safely.