can subs attack other subs?
if both sides have submarines present in a naval battle with destroyers, the subs must participate in combat. however, as soon as a side has removed all destroyers as casualties and their opponent still has submarines, they may submerge and escape the battle if they wish.
Therefore, during the combat movement phase, a power may choose to attack a lone sub with a lone sub, but during the resolve combat phase the defender may choose to defend and resolve combat or submerge before any dice are rolled (the defender is not required to make that choice until all combat movements are finished).
Remember this: Submarines have 3 special abilities,
1. Submerge to escape combat
2. Pass through hostile sea zones within their movement limit
3. Surprise strike disallowing casualty rolls
A submarine may not do any of these with an enemy destroyer present.
In my experience, people forget to use their own subs special abilities, once their opponent removes their destroyers in the early rounds of the battle.