In Axis & Allies 1940 edition, submarines are able to make a surprise attack in Sea Battles, allowing them to score hits before other units get to roll. An opponent can bring a Destroyer to nullify this unique ability of subs.
However in the current implementation of Global 1940 version 3.7 (and perhaps others), when a DD is present on the enemy team, the submarines will still roll their dice (thus informing both players how many hits they will score) before other units roll.
Steps to reproduce:
1. Attack a sea zone containing enemy submarines with a force containing one or more Destroyers.
2. Notice the defending submarines will roll in the surprise attack phase, instead when the defenders normally roll.
Expected result: Submarines should roll at the same time as the rest of a force when enemy Destroyers are still alive in the battle.
Per the Axis & Allies Europe 2nd Edition PDF, pg. 19, under "Step 2. Submarine Surprise Strike or Submerge": "However, if the opposing side has a destroyer in the battle, the attacking or defending submarines can’t submerge or make a Surprise Strike. Combat proceeds normally with your submarines firing along with your other units in step 3 or 4."