Since I wrote this already, I might as well post it
it contains same info as Battuta and Dias, but with MORE WORDS
You have everything you need.
The best way to play the FreeSpace: The Great War campaign is by using the FS2 Open game engine to run the FreeSpace Port.
To accomplish this:
1. Install FS2 on your computer, preferably on some other location than C:\Program Files\ as that can sometimes cause some problems. Something like C:\Games\FreeSpace2\ will probably work better.
2. Set up FS2 Open (Launcher and fs2_open.exe) and, if you are so inclined, MediaVPs (which you could think of as HD remake of original FS2).
3. Install FreeSpace Port and, again if you so wish, the Port MediaVPs (which as you might guess are HD remakes of the original FS1 files).
Steps 2 and 3 may need expanding but I'll leave that for someone else as it's getting late and I should go to bed...
Acronym legend:
FS1 = FreeSpace: The Great War
FS2 = FreeSpace 2
FSO = FS2 Open = FreeSpace 2 Open
FS Port = FreeSpace Port
FS SCP = FreeSpace Source Code Project
VP = Volition Package, a proprietary container file format designed by Volition, the developer studio of FreeSpace series.
EDIT: Oh, and I forgot: You mentioned the Silent Threat expansion for the FreeSpace 1?
Well, HAVE NO FEAR for it has also been re-done for the FreeSpace Port and it is in many ways more fabulous than the original. It's called the Silent Threat: Reborn. It should be played between FreeSpace: The Great War and FreeSpace 2 campaigns, chronologically speaking.