Deuteronomy 3:8
And we took at that time out of the hand of the two kings of the Amorites the land that was on this side Jordan, from the river of Arnon unto mount Hermon; And we took at that time out of the hand of the two kings of the Amorites the land that was on this side Jordan, from the river of Arnon unto mount Hermon;
The land taken from the two Amorite kings lay east of the Jordan and stretched from the river Arnon to Mount Hermon. The two kings were Sihon and Og, encountered as Israel advanced northward toward Bashan. That territory contained numerous fortified cities built of stone and formidable inhabitants, including Og himself, so Israel faced great human obstacles in taking it.
Those earlier victories over Sihon and Og were followed by the crossing of the Jordan. Together they showed Israel and the surrounding nations that God was with His people and would not fail or forsake them. The news of what God had done filled the nations with fear.