Rashi on Deuteronomy 3:12/רש"י על דברים ג׳:י״ב

1ואת הארץ הזאת AND THIS LAND mentioned above (v. 8), "from the brook of Arnon to Mount Hermon", ירשנו בעת ההוא WE POSSESSED AT THAT TIME.

2מערער אשר על נחל ארנן FROM AROER, WHICH IS BY THE BROOK ARNON — this must not be connected with the first part of this verse (defining הארץ הזאת) but with its conclusion — with נתתי לראבני ולגדי I GAVE TO THE REUBENITES AND TO THE GADITES; however, as respects possession (taking by conquest) that was "[from the brook of Arnon] to Mount Hermon" (v. 8).