"And in Seir the Horites used to dwell, and the children of Esav dispossessed and annihilated them and dwelt in their place, just as Israel did to the land of its inheritance which God gave them." (Devarim 2:12)
The obvious question here is: didn't the Jews conquer the land of Israel in the FUTURE? After the Torah was written? So why is this written in the past tense? Isn't this one of those scary anachronism things, you know, the ones that show that the Torah was written by a circle of monks in the 12th century and not by God at the time of Moses?
Well, not necessarily. You can make a very good case that the "land" under discussion is not modern-day Israel, but the kingdoms of Sichon and Og which were conquered at most eight months before Moses' speech here.
(For background, the story of that conquests is in the second half of Bemidbar 21.)
In our verse, exactly what identifying information do we have about the land under discussion?
First, it's a land that God GAVE to Israel. Second, Israel DID various stuff to that land. What exactly did they do? Well, exactly what Esav did - that is, DISPOSSESS and ANNIHILATE its inhabitants, and DWELL in it. Last, the land is THE INHERITANCE of Israel. (Note: to "dispossess" and "inheritance" are different forms of the same verb - "lareshet". I suspect that verbs from this root can be correctly translated based on whether their subject is a people or a territory. Nevertheless, it's clear that dispossession and inheritance are closely related.)
Looking in Bemidbar, we see these exact words used to describe the conquest of Sichon and Og.
"And Israel smote him [Sihon] by the sword and dispossessed/inherited his land..." (Bemidbar 21:24)
"And God said to Moses... I have given him [Og] and all his people and his land into your hand..." (21:34)
"And they smote him [Og] and his sons and all his people until no remnant was left, and dispossessed/inherited his land." (21:35)
The words DISPOSSESS/INHERIT, DWELL, and GAVE (land, by God) all appear here, just as they do in Moses' later description. The only key word which does not appear is ANNIHILATE. But read the three verses I just brought, and try convincing me that annihilation is NOT exactly what Israel did to Sihon, Og, and their peoples. The verse in Devarim, then, contains pretty much a perfect summary of the stories of Sihon and Og.
Furthermore, that verse does NOT even fit that well with the conquest of the "real" land of Israel. While Joshua and company pretty quickly take care of the 31 leading kings in Historical Palestine, finishing the conquest is a much harder task. At the end of the day, there are large Canaanite populations remaining in various parts of the land, and other parts are not conquered at all (see Shoftim chapter 1). Thus, the words DISPOSSESS and INHERIT are not perfectly applicable, especially when you apply them to the entire land of Israel which was GIVEN to the Jews. But regarding the kingdoms of Sihon and Og, all sources seem to say that the conquest was much more complete, which fits our verse much better.
UPDATE: A little more evidence on the subject. Devarim 2:24, 2:31, 3:12, 3:20 (all very close to our original verse) all use the root YaRaSh (=dispossess/inherit) to refer to the east bank.