Deuteronomy 1:4 (ESV)

4 after he had defeated Sihon the king of the Amorites, who lived in Heshbon, and Og the king of Bashan, who lived in Ashtaroth and in Edrei.

Israel has already experienced the beginning of the conquest of the land, with victories over Sihon and Og in the region beyond the Jordan. Such victories are recorded in greater detail in Deuteronomy 2:36–37 (Sihon) and Deuteronomy 3:1–22 (Og).