[Trevally.]

Originally Posted by gap

Okay, since many words didn't serve, I will try explaining it with a diagram:

... ... = CRASH

In that mission, Italian ships are your allies. You simply can't sink them

Originally Posted by acecol

The problem here, is it a bug in the UI or built in to the UI.

It is a bug in the UI
You can fix it by not killing your own side