You will find that in Galactic Scourge the security protocols vary with governments. The Confeds, Rebels, and Pirates are pretty strict, while the planet-states are less organized. There are also trigger missions. For example, you will not always get the "Save the Universe" (typical Human overstatement) mission even if you have followed that wicked path. The same is true with the destruction of New Cydonia. It is very easy to go part way, just get the technology, without the messy legal battle that follows, depending on what seemingly unrelated mission you have done.
In the specific case of Diphidia/Astex, one can progress through the missions as a double agent (if you keep both planets liking you) to a point, then the security impoves and you are locked onto one side or the other. If you have destroyed Diphidia, then you chose one path. The destruction of Antares is unrelated to any Diphidia action.
Detailed information about missions can be found at (url="http://"http://www.geocities.com/ev_scourge/cheaters.html")http://www.geocities...e/cheaters.html(/url)
Enjoy.
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-Jason
(url="http://"http://www.geocities.com/ev_scourge/")www.geocities.com/ev_scourge/(/url)