Do ally destroyers prevent the submersion of submarines?

@Admiral_Yamamoto:

I have question in AAR. Do submarines that belong to a nation allied to your own prevent enemy submarines from invading. Example: Germans move a submarine next to England during their non combat phase. The English place a destroyer in that sea zone in their place units phase. An American bomber goes and attack the German submarine duriong their combat phase, while the the British destroyer is present. Does that prevent the sub from submergeing if the Bomner misses the target?
thank you for your help.

Short answer is No.

Attacking units are always of the same country (except for the D-Day NA).
US bomber can not bring the UK DD into battle, they do not control that UK DD.

Axis_roll is correct.  To quote from LHTR 2.0:

Any number of attacking submarines may withdraw by submerging. They may do so after Step 6
in any cycle of combat. If they submerge, other attacking air and sea units are not required to
withdraw at the same time. Submarines may not submerge if opposing destroyers are still on the
battle board

Emphasis added to show that the DDs must be part of the battle itself (“on the battle board”) not just sitting there, admiring the glow of the battle from the other side of the sea zone.

A related topic brought up a major discussion about 2 years ago…  The result was LHTR 1.3 that effectively stated that any units of your side that happen to be in a sea zone you are attacking do not exist (they are elsewhere in the sea zone).

ALL enemy defender units are present, but any units from your side that are there when you move (a contested sea zone) are ignored completely for combat… they cannot attack, they cannot take hits, and DSTs don;t prevent sub submergence.

So, with the D-Day N A, would the DD from the UK partake in the melee with the Bmr on the U-Boat, or just keep it from getting away under the waves?

@Crazy:

So, with the D-Day N A, would the DD from the UK partake in the melee with the Bmr on the U-Boat, or just keep it from getting away under the waves?

I believe in Joint Strike (LHTR name for D-day), the US commandeers (takes control) of all UK units and they act as one force.

In this case, the DD WOULD attack the sub  as well as prevent the sub submersion.