Why do so many of you people assume that just because the UEF didn't expect the GTVA to be hostile towards them, that they would treat the Shivans the same?
The GTVA are their long lost brothers and sisters. They (or half of them anyway) are the same species, while the Shivans are a species that very clearly demonstrated it's hostility, unwillingness to communicate and intent to completely wipe out the Human race in the great war.
The UEF might be peacefull, but they aren't total pacifists. They have a quite massive fleet and ships that outmatch any other ships they every saw before the node was reopened with the sole exception of the Lucifer. And the contact between the 14th battlegroup and the Renjian pretty clearly shows that they are not shy to employ violence to defend themselfs.
Also the UEF government doesn't just tolerate, but actively encourages discussion and critique of themselfs. To me that sounds like their form of government is well siuted to quickly adapt to changing situations.
So the GTVA was able to collect this information via the peliminary probes of Sol? I realize that some time has passed since the events of AoA, but the dossier says that this was the basis for the GTVA's decision to go to war with UEF.
With the exception of the Fedayeen (about which the GTVA knows pretty much only that they exist) there is really no information in that dossier that wouldn't be freely available to every UEF citizen. Look at our own political systems and history.
With the exception of some stuff that's kept under cover and the occasional manipulation of the truth everything is freely available in books, internet, TV, radio, newspapers and so on. If the GTVA would send probes back in time, they'd just need to access the internet to get as much information as they have on the UEF (though they'd probably have a harder time telling what are facts and what are fiction, rumors or outright lies).
And considering that the Fedayeen are most likely somewhat controversial within the UEF, I wouldn't be surprised if there were quite a few discussions and acusations thrown around in the media and whatever succeded the internet in the 24th century. Or course such "sources" only give you rumors and no hard facts, but then the dossier states quite clearly that the GTVA has no clue what the Fedayeen really do.
"they appear" "it is rumored" "it's function is unclear and puzzling" "We are not certain".... four times stating their uncertainty in just three paragraphs shows pertty well how little the GTVA know for sure about the Fedayeen.