Is ethnic cleansing ever acceptable?

Chris and I have often talked about the old testament era when Israel occupied Canaan, mostly by a process of ethnic cleansing, although in some cases making the indigenous people slaves instead. It was a nasty episode in a nasty era, and we really have little idea of just how foul and unpleasant people could be. It was God's judgement on a group of peoples whose way of life needed to be stopped.

One of the debates we've had is whether there is a similarity between what was done then and what happened to Monteczuma and the Aztecs. Hugh Thomson has an interesting piece in the times about how the actions of Cortez were beneficial to Mexico, and just how deeply unpleasant this particular kingdom was.

Now whether one evil regime was replaced with another even worse or not is open to considerable debate, but it does make me wonder about the appropriateness sometimes of old-testament style judgement in a new testament era. I don't know why, but God seems more offended by massed shedding of blood for religious purposes than for war or even greed. I wonder if the sheer calculated religious evil of this people is what brought relatively swift judgement upon them?