Proposal Alien type : Thrall
Gray-skinned,short,stubby aliens which construct small,pylon-like structures in remote (mostly mountainous) areas.Hard to discover.However,some governments realize to their horror that the tiny pylons emit high radioactivity,and a deep scan in the area shows that the pylons are draining heavier,radioactive materials at a very fast rate from earth.It is apparent that the Antareans have begun small-scale exploitation zones to fund their war-effort,as materials like Uranium or Thorium are (I don't know if true) rare in oter areas of our solar system.
Even if they are not rare,imagine this as the gold-rush of North America,they the White Men in individual "New World Trading Companies",YOU the Indian.Scary now,right?UGH!
Discovered pylons should make Governments REAL ANGRY unless you demolish it fast.And demolishing it requires an assault,which will be resisted by a large group of Thralls which operate the tachyon mining system(OK its fiction I know),a few Antarean Soldiers,and an Antarean Commander (acting as Governor of exploitation area)
Thralls use basic plasma weaponry,but are resistant to earth based bullets,also especially flamethrowers,since they are indigenous creatures of a Volcanic planet.They don't like meleé,they are not even ferocious warriors.They just do the job for their Whitey...err Antarean bosses.