It was, I quote, 'a short time before [the invasion's execution], but not necessarily just a few days. There are examples of this throughout history, even from nations that had decent intelligence capabilities - big things get missed because they seem crazy, or because the necessary information is spread out across too many desks. I doubt the Vasudans were unaware of what was about to happen, I'm sure they saw signs, but final confirmation came late. Given how chilly relations between the two powers were there was likely little pressure they could exert.
I'm sure they were furious - but when it became clear the GTA wouldn't back down, Khonsu eventually looked for another option: end the war as rapidly as possible.
Actually, here I've got to disagree with you. These are all signs of good intelligence groundwork on the part of the Vasudans. Firstly, they understood that the Gaian Effort is not monolithic, there are a lot of factions with a lot of different agendas. Second, and more damning, is something that wasn't made very clear in R1 - Federation Intelligence made an effort to turn Gaian Effort factions against the GTVA earlier in the war, even going so far as to supply weapons and ships.
The Vasudans would see this as evidence for UEF attempts to use the Gefs as a catspaw, thus laying the groundwork for Steele's plan to draw them into the war.
Not incapable - just selective. Steele was able to spin their interpretations of the facts the way he wanted.
Some of the techroom fluff describes how the diplomatic rift has even extended to the Security Council, really hampering the flow of information. I'm sure they were furious, but they're also expert at realpolitik. Better to seek forgiveness than ask permission, the GTVA figured.
I concede the point that intelligence services have sometimes missed major operations, but the good ones, the capable ones tend to learn from their mistakes. The Vasudans already know that the Terrans have concealed information from them and lied to them about the invasion which is not trivially diplomatically and a lesser alliance would've fracured on that alone. Further the relationship between their two powers have been deteriorating for the last couple of decades which this invasion isn't helping. And yet the Vasudans implicitly trusted Steele and Terran intelligence to tell them the truth in a situation where clearly it would be in the best interest of the Tevs to lie and fabricate. Even when I was listening to that conversation during the mission, I couldn't believe that anyone, much less a Vasudan where the bio in FS1 stressed the reliance on intrigue in their society, would be so naive.
That the Gaians are not monolithic and that Fed intelligence tried to court them both make sense and I can see the Vasudans using that as a basis for not trusting the UEF (as well as the incident with Vasudan logistic vessel). But frankly at this point I wouldn't be trusting anyone if I were the Vasudans - especially not my Terran allies. Again, to me, Vasudan intelligence have missed two major pieces in a row ... and I seem to recall an expression about fooling someone twice ...
They could have done a lot more, though - their peacetime infrastructure was a match for the whole GTVA due to how long Sol has been inhabited.
I think there was just a discussion of this here in this thread. The fundamental argument the GTVA has is that the existence of the UEF will lead to the collapse of human starfaring civilization as colonists undertake a massive return to Sol - fleeing from the cold and the dark and swamping the UEF with a population boom it may not be able to endure. The GTVA will be unable to stop this migration without triggering rebellion, and thus it will be unable to provide a military defense against the Shivans and there will be no chance of stopping a new Shivan incursion. They reached this conclusion from psychohistorical projections, and they moved to stop it.
The GTVA believes that if the UEF doesn't go down, humanity will.
What's important isn't that you necessarily feel the GTVA it's justified, it's that you feel the GTVA feels it's justified, whether through genuine concern or self-interest.
The GTVA already saw itself divided by the mere promise of building a new Earth. The threat of an actual Earth drawing its citizens into rebellion is just too much to countenance.
Placing Sol under GTVA control allows the GTVA to dictate the terms of that exodus and shape its consequences. Liberating the ancient homeland is a much more appealing narrative than grudgingly allowing its citizens to return to a glorious paradise state.
And the GTVA's gamble was that the war would actually pull its citizenry together. It's not clear whether this has paid off.
Well everyone thinks they have good reason. To say that everyone believes they are right is almost a truism and I don't actually think that trying to show that explicitly actually gets you anywhere or sets up a moral conundrum unless the reasons have equal weight. More important is to show the basic humanity of the enemy coupled with the horrors of war you both inflict, which you did an excellent job of. That is what reminds us much more than reasons that sometimes being enemies is simply being born on opposite borders or to opposite points of view. My disagreement over the existence over a moral conflict is not whether or not both sides think they're right, but rather whether or not someone's arguments for why they are right are logically internally consistent (have validity) or whether or not the assumptions held in their arguments hold up to reality (have veracity). Veracity can be very hard to prove, especially in issues of morality where people's moral spectrums often differ and cases can be interpreted in many ways. That said I do not believe the Tev's logic and action is even valid - i.e. is not consistent.
Having control over both sides or one side of the gate does not change the basic equation in their model that in controlling the flow of population, people might turn against them and rebel in some mass attempt to get to Earth. Having control over the egress simply doesn't change that - it really doesn't help you. If they were truly concerned over this possibility, then the optimal solution would be to never have finished the gate project to Earth in the first place or to have it take so long that people forget the drive to go home. It's already been 50 years, almost two generations would have been born never having had a connection to Earth. Simply continue to cite technical difficulties or expense and allow the project to slowly die or only have success at a time when the drive to return home is minimal. The second best strategy, if they must open the gate and have a war, is not in fact to win said war. Draw the war out to a stalemate, eventually thawing relations, and use that as their excuse to allow minimal civilian traffic through the gate. Actually winning the war, especially quickly, should be the last thing they would want to do if worry over an exodus from their space is their motivation. Because then they'll still get the exodus after winning and then have no excuse not to stop it. Having someone else control the egress actually gives you an excuse over why you can't let too many people through and pass the blame.
I also don't find the Tevs rationale valid - i.e. the assumptions of their decision don't seem to resist the reality of their situation. Although here I run into problems since you know the game-world far better than I since you the Blue planet constructed it whilst I get the boiled-down Tech room version. For instance you say that the UEF could've built a bigger fleet whereas from the information I see from the game, I'm simply impressed that they built the fleet they did without an immediate military exigency to do so, without access to Vasudan/Ancient tech, with the resources of only one system, and with a much higher concentration of population to support from that one system's resources. As such I am willing to admit that my arguments over validity may themselves lack validity since I didn't create the game world and thus don't have as much access to information about it as yourself. However, with rather large caveat in mind, here is how I see it:
1) You yourself point out that the UEF might not like the idea of a flood of people overwhelming them and that the Tevs thought of this. Therefore the UEF would probably be more than happy to help control gate flow thus the blame would be shared.
2) One of the stated reasons for why the Tevs don't simply crush the UEF with overwhelming force is that susbspace travel between Delta Serpentis and Earth is energetically expensive, presumably at the energy cost of running the Terran copy of the inter-system gate. Thus they can't supply and send in huge fleets at a whim. That in itself should provide some physical limits on how many people could conceivably go through at any one time.
3) It is just as likely for the humans in Sol who have been bottled up in that system for 50 years to want to explore brave new worlds. The exodus away from Earth might match or even exceed the one trying to get to Earth. Such a flux of population might not be in either side's best interest and would reinforce that it should be in both of their interests to control gate flow. Thus the UEF would have almost certainly helped to keep gate traffic manageable even if #2 didn't do that by default.
4) To invert Bosch's monologue describing himself, the NTF rebellion was born more out of a hatred for Vasudans than a love for Earth - it thus seems difficult to apply the same rationale for the NTF rebellion as the possible rationale for a possible rebellion to get to Earth who themselves may not want any association with that rebellion. The Vasudans and BETAC treaty don't really enter the possible rebellion's equation to me. In fact if anything, quite the opposite. After all the UEF seems very accepting of Vasudans and even describes their culture as inherently similar to Vasudan culture. So it would seem that contingent of Tevs would be rather sorely disappointed in hoping to find an all human refuge away from the meanie Vasudans.
5) This is a weaker point since one can never really know how something will go, but the potential instability in their own population and fleet caused by their actions is also huge as evidenced by the defections of several ships, crews, and certain key personnel in the first invasion force. Again that points to your statement that they were aware that this was a gamble.
Thus as someone who recognizes that inherently everyone thinks that they're right, I don't feel that beyond that statement there seems to be much rhyme or reason to the Tev's action of starting the war with the UEF. Morally there doesn't seem to be a conundrum. However, having the GTVA (or at least the GTA part) be the bad guys is a nice twist.
Glad you liked it!
Absolutely. Again please take the above criticisms as a sign that I feel that you guys in the Blue Planet team created in your WiH game a (mostly

) well-thought out game world and a thoughtful, emotionally visceral story with complex and interesting characters which I found truly fascinating to think about. These are emphatically
not criticisms for why I didn't like your story or game world. I can't wait to see what you do with Release 2. Oh I forgot to mention that one of the things I liked about your game was the title.

I know others have mentioned it already, but it is very fitting and evocative for a space combat sim - especially with your story.