Deuteronomy 3:21

And I commanded Joshua at that time
After the conquest of the two kings, and the assignment of their countries to the above tribes; and after Moses had it made known to him that he should quickly die, and Joshua should be his successor; then, by the direction of God, he gave him the following charge:

saying, thine eyes have seen all that the Lord your God hath done unto
these two kings;
Sihon and Og; how their kingdoms were taken from them, and given to Israel, and they slain with the sword; this Joshua was an eyewitness of, and was, no doubt, greatly concerned in the battles with them, being the general in the Israelitish armies; at least this was sometimes his post, and he cannot be thought to have been unemployed in these wars:

so shall the Lord do unto all the kingdoms whither thou passest;
all the kingdoms in the land of Canaan, where there were many, thirty one at least; these would be all conquered and put into the hands of the Israelites, and their kings slain.