Looking over an index of the wars we know of in Tamriel's history, something stood out to me: Most of them seem to be tied to the rise of an Empire or during the reign of an Empire. Yet many will say that the Empire is keeping the peace in Tamriel when they are in charge.

So what does the lore forum think; is it a force for peace in Tamriel, or is that propaganda to justify ruling other nations?

A little from column A, a little from column B. And it all depends on what you mean by peace.

I think this is the problem with nations and the like - they will use force to keep people (and in the case of the Tamrielic Empire, independent states) in line with their wishes, but they also prevent some of the wars between their constituent parts. Particularly, there wasn't any open conflict between Black Marsh and Morrowind until the Empire weakened (that I know of, someone correct me if my history is out of whack), and the Empire saw off two invasions from Akavir.

In the case of the current Septim empire, such as it is, is helping people (or at least, humans) by stopping (at least temporarily) the Thalmor obliterating Talos and thus getting rid of humanity. The Reman empire struck me as a bit more overtly political, but that might be because it had to be more Machiavellian in order to sustain itself. Is this justifiable or not? I don't know.

On the other hand completely, I'm sure a Thalmoric Tamriel (or Mundus) would be peaceful in a way. Would it still technically be an empire? I don't know.

Thinking in terms of historic parallels (always dangerous with TES, but bear with me), empires break down when national consciousness outweighs the benefits of empire for those holding power on the ground. Generally the ruling classes of the various provinces are ignored by the Empire until trade is affected, so I'd say things are close to breaking up (and always have been), but there's still a few minutes before midnight; there's a provincial/racial consciousness in Tamriel, but not a national one. Whether this will (or can) evolve on Tamriel is an open question. This is down to the resolution of fantasy tropes as much as anything else; modern nations aren't really de rigeur for fantasy.

when the empire was able enough, it was yes, now? not so much

their gradual disintegration has plunged many of its ex-provinces into chaos