Who are the colonizing aliens?

Before planting a big wet one on Mulder's cheek, renegade agent Alex Krycek (Nicholas Lea) notes there is an alien "war raging" and that humans are facing "the colonization of this planet by an extraterrestrial race." But we all know there are any number of aliens in the X-Files universe. So who are the bad ones?

Fans seem divided. "The shape-shifters are the colonizing aliens," says fan Sheila Bodey via E-mail. "They are attempting to coexist with the people of Earth, but they also are not going to take no for an answer." That would explain why the shape-shifters seem to be in bed with those nasty old codgers, The Syndicate. The other school of thought says that the black oil is actually a sentient alien species and not just an otherworldly virus. That would make the black oil the colonizing aliens. And that's why everyone is so desperate for a vaccine against it.

Which leaves the question of what these two alien races have to do with each other. "We've seen these shape-shifters and they seem to be enforcers for alien law," explains Spotnitz. "We've also seen this black oil that can go inside people and make them subject to its control." Which suggests, though Spotnitz won't elaborate, that the shape-shifters are in thrall to the black oil. Uh, doesn't it?