Attacking subs have the “first strike” capabilities.
Each sub gets to roll first (in EVERY round) when attacking. Any hits are removed and the killed units do not get to defend.
Germany attacks the UK navy with the following:
UKSZ – UK 1 bb, 1 trn RUS 1 sub, 1 trn vs. GER 2 sub, 3 ftr
GER rolls a 1 and a 3 – thus, 1 sub gets a hit and the Allies wish to remove the RUS trn
Next … GER finishes the rest of it’s rolls (3 ftr) … and roll 2, 1, and 3 (3 hits). The rest of the Allied navy is sank.
Allies get to defend with only 1 bb, 1 trn and 1 sub (the RUS trn was sunk via the GER sub’s first strike ability so it doesn’t get to defend at all).