Well, time for another crazy theory 
The barnacles are some type of serious space plague. They settle down on planets, spreading there until the planet is totally destroyed / useless, then move on to the next.
The Thargoids know about them, because the barnacles have already destroyed a large portion of their home galaxy (which is LMC if i recall correctly?).
Now the barnacles have come to our galaxy and beginning to infect us.
The UAs are actually from the Thargoids, trying to help us. They set up the shell to contain the barnacles inside so that they can no longer spread.
This will finaly lead to an alliance with the Thargoids to perform a massiv war against the barnacles!
No matter what, there will be war in the end...
The barnacles are some type of serious space plague. They settle down on planets, spreading there until the planet is totally destroyed / useless, then move on to the next.
The Thargoids know about them, because the barnacles have already destroyed a large portion of their home galaxy (which is LMC if i recall correctly?).
Now the barnacles have come to our galaxy and beginning to infect us.
The UAs are actually from the Thargoids, trying to help us. They set up the shell to contain the barnacles inside so that they can no longer spread.
This will finaly lead to an alliance with the Thargoids to perform a massiv war against the barnacles!
No matter what, there will be war in the end...