Making Submarines acting like Privateers seems easy enough, there's a <bHiddenNationality>0</bHiddenNationality> value in the unitinfos.xml that, turned to 1, should do the trick (I'll try later tonight). Edit: Actually it's not as straightforward as that, I see other civ's subs with the pirate (barbarian) flag, but trying to attack them prompts the "do you really want to declare war" popup. And using my subs to try and attack another type of ship also asks to declare war. Well, silly me, right under the hidden nationality tag there's also <bAlwaysHostile>0</bAlwaysHostile> which, turned to 1, solves the problem! The actual question is: is it possible to make it so that there is a chance, every time a hidden nationality sub attacks a unit of a civ you're currently at peace, to be discovered? And have negative diplo consequences, or outright war, like with spies? I guess this would actually be interesting with privateers as well, so far they rule the seas until ships of the line and iron frigates appear (and, promoted enough, my privateers still sink most of the SotL anyway).