Numbers 32:21

And will go all of you armed over Jordan before the Lord
Moses tries them thoroughly, and is very express in his words, requiring them not only to go armed, or march from the place where they were, towards the land of Canaan, but to go over Jordan, and not some of them only, but all, and that before the Lord; though indeed, when the tribes came to the river Jordan, the ark, which was the symbol of the divine Presence, went before all the tribes into it, and there stayed till they passed over, and then these two tribes and the half tribe of Manasseh passed before the children of Israel, and before the Lord, unto battle, ( Joshua 3:11 Joshua 3:17 ) ( Joshua 4:12 Joshua 4:13 )

until he hath driven out his enemies before him:
the Canaanites, who were the enemies of the Lord, as well as of his people; and because of their sins, in which they showed their enmity to God, the land spewed them out, and he drove them out to make way for his people Israel, and till this was done the tribes of Reuben and Gad were to continue with them.