Not sure what you mean. A full round of battle most occur before you choose to retreat.
A. You can’t sneak fire and withdraw right after. Other attackers and defenders have to roll.
B. After a full round must occur : Sneak, remove casualties, attacker fires, assigne hits, defender fires, assign hits, remove casualties… and then, the attacker choose to withdraw or carry one with another round.
Furthermore, say you didn’t have a DD, I must state before attacker rolls if my sub submerge or not. So, for instance, my sub survives first round and your DD don’t, at start of 2nd round (taking you choose to continu), I can submerge my sub.