Numbers 32:20-21

If ye will go armed before the Lord— i.e. Before the ark, which was the symbol of the Divine presence; for the three tribes of Reuben, Gad, and Simeon marched immediately before the sanctuary, ch. Numbers 2:10; Numbers 2:14 Numbers 10:18 so that Moses requires of them only to hold their usual place when they went to war against the Canaanites. All of you, in the next verse, signifies, all of you who shall be called upon to go; for they were in the whole above one hundred thousand men able to bear arms, adding the half tribe of Manasseh to the tribes of Gad and Reuben. See on Numbers 32:17. To encourage them to go, Moses represents the idolatrous Canaanites as the enemies of Jehovah; upon whose protection, therefore, they might certainly depend, until he hath driven out his enemies from before him.