Numbers 32:17

But we ourselves will go ready armed before the children of Israel, until we have brought them unto their place: and our little ones shall dwell in the fenced cities because of the inhabitants of the land.

Because of the inhabitants of the land. There was good policy in leaving a sufficient force to protect the conquered region, lest the enemy should attempt reprisals; and as only 40,000 of the Reubenites, and the Gadites, and a half of Manasseh, passed over the Jordan (Joshua 4:13), there was left for the security of the new possessions 70,580 men, besides women and children under twenty years (cf. Numbers 26:17).