You have to purchase Entrenchment to purchase Diplomacy, but do you really need Entrenchment?  What would happen if you deleted "Sins of a Solar Empire Entrenchmnet.exe" from your computer and then loaded up Diplomacy?

Actually, you do, in the same way that Entrenchment needs the base game.

Nothing good, I think.  You're welcome to try it, but don't say I didn't warn you...

Now that I think about it, that probably makes sense.  All the data is still there for Entrenchment, just not the .exe file.  Fair enough, ice.