E. SUBMARINE: Submarines may only fire on sea units. Submarines do not make a sea zone hostile to enemy units. Submarines are capable of making a first strike both when attacking and defending by firing before any other sea units. Hits are taken immediately and destroyed enemy ships may not return fire (unless it is also a submarine making a first strike). Enemy destroyers cancel this ability for the duration of the battle on a 1-for-1 basis.
Example: 5 submarines attack 3 destroyers and 1 cruiser. 3 of the submarines are detected by the 3 destroyers automatically. The other 2 subs can either stay in the battle and get a first strike or may retreat to the nearest friendly sea zone. These 2 subs can retreat only because they cannot submerge in a sea zone that contains a destroyer. Even if the preemptive strike destroys 1 destroyer, the 3 submarines that were detected are still detected and cannot retreat or submerge.
Additional submarines may be detected after the first round of combat if the destroyer-to-sub ratio changes due to combat (more subs are sunk than destroyers).
Example: After the first round above, let’s say 2 subs are sunk but all the destroyers survive. For the second round of combat, if all subs stay in the battle, all 3 remaining subs would be detected and none would receive their first strike.
Undetected submarines may only be fired upon by enemy submarines. Undetected submarines also have the ability to submerge and leave the battle, provided there are no enemy destroyers left. They can do this instead of using their first strike. Submerged submarines may not return to the battle at a later time.
We will clarify this in the 6.0 rules.
If 2 subs attack 2 cruisers. The subs get a first strike, or sneak attack, shot and casualties removed without a defending shot. Since the subs fired they are detected and any remaining units may return fire. Now on the second round of combat, and any round after, the subs still get a sneak attack or they can submerge and end the battle, because there are no destroyers in the battle.