Can you link me the spoil to this you mentioned earlier, would help?
It isn't outright stated, but you should be able to piece it together, if you read the following posts:
The Dread Lords, the Arnor, the humans, and the Altarians
The Galactic Civilizations Universe
Also, stop blaming the Thalan. They tried to prevent it, but the humans didn't want to listen. Here is the description of the last mission in the DL campaign:
Earth is safe -- thanks to the Salvation Device. It has surrounded the Earth in an impenetrable field. While the Drengin have Earth surrounded, they cannot invade. And we have custody of the Salvation Device.
Now is the time to escape. But where should we go? The mysterious figure has arrived again. He... It... continues to wear a long dark robe. As he approaches, Izok of the Thalan arrives in a shuttle. He has insisted that we not talk to the mysterious figure!
Izok claims that the mysterious figure will lead the entire galaxy to ruin. In response, the mysterious figure unrolls his hood revealing himself -- he's human! He calls himself "Draginol".
To us he says, "The galaxy is shattered, crushed by the evil Drengin and their allies. But the real lords are the Dark Arnor -- the Dread Lords. You have but one hope. You must return to Karion. On Karion is the temple with the device that brought the Dread Lords here. It is too late to send them back, but the device can send you and your forces to where the Dread Lords were. Think of it -- the Dread Lords inhabited that place for centuries, the power and devices that must still be there are the only hope for the free peoples of the galaxy. You must go there and in time, return. Your return will be the beginning of a great crusade to rid the galaxy of evil and tyranny and put an end to the Dread Lords once and for all."
In response, Izok disagrees, "No, that is what we have been trying to tell you. The Thalan are not of your universe. Humans have interefered too many times in the affairs of the galaxy. Your Hyperdrive technology was only the first step. As bad as it is, there are worse things for the universe as a whole than one galaxy dominated by these Drengin creatures. Humanity has been manipulated from the beginning by greater powers. Without your Hyperdrive technology, none of this would have happened. What this 'Draginol' suggests will lead to greater ruin."
If you go to Karion and to the place of the Dread Lords I have no doubt you will return. And your "crusade" will end in fire and death for all living things. That is why we are here, we were sent to stop you. But events occurred too fast and now it's too late so I can only appeal to your... humanity. You must not do this thing."
The Draginol figure looks vaguely familiar though you cannot quite remember from where. However, the decision as to whether to leave the Earth imprisoned forever or to find hope for the future is a no-brainer. Earth WILL be saved. We will go to Karion.
Draginol explains that at the temple we must bring our ships as close as possible. Once the device is activated, our ships will be teleported from our universe to the pocket universe that the Dread Lords were in.
Karion is occupied by the Dread Lords. We must take it back.
Maybe they should have sent a few intelligence agents into the past to sniff things out first.
The Thalan method of time/dimensional travel is a one-way trip, as far as I know.