In the past is seemed like the Thieves Guild was an unofficial Imperial backed faction, one that was allowed to exist within the Empire. In Skyrim the Thieves Guild definitely has the backing of Ms. Black-Brier, who apparently supports the Empire (and is made Jarl if the Empire wins the Civil War).

But the Thieves Guild also seems less organized and a bit like bullies and ruffians (as they are described by the Lioness).

So, are they still an unofficial Imperial faction? If I join them would it make more sense to work for the Empire during the civil war, or are they so far away from anything respectable anyway that it really does not make a difference?

Thanks.

I don't think they were backed or even sanctioned by the Empire. Tolerated is more the word. Tacitly permitting the existence of the TG meant that crime was somewhat self-regulating. This is advantageous in a world where any street urchin can get his hands on an invisibility scroll.

The state of the TG in Riften reflects the state of the Empire itself. It makes sense that the TG thrives when commerce thrives. The decline of Imperial power most likely means a general decline of the economy and thus, hard times for the TG. In a town like Riften with a long tradition of corruption, it's no surprise that a bully like Maven would manipulate the TG for her own purposes. Indeed, she even has the Dark Brotherhood wrapped around her finger. For Maven, both factions are just tools of power.

SPOILERS!!!

The sad sate of the TG had more to do with the closure of the Twillight Sepulchur than the sad state of the Empire.