‘ And the children of Reuben, and the children of Gad, and the half tribe of Manasseh, passed over armed before the children of Israel, as Moses spoke to them, about forty eleph ready armed for war, passed over before Yahweh to battle, to the plains of Jericho.'
This is not chronologically placed. The point is that they passed over at some stage in the sight of the people and in the sight of YHWH, ready armed for the coming battle, just as the ‘Ark of YHWH' had crossed over ready for the battle. This was not just a crossing over, it was a crossing over with a martial purpose.
“To the plains of Jericho.” These were in striking contrast to what they had left. The plains of Jericho were well watered and fruitful, with rich soil, a greenhouse climate and irrigated by perennial waters.
“Forty eleph.” That is, forty fighting units (the word eleph means ‘family', ‘clan', ‘sub-tribe', ‘military unit', ‘captain', or ‘thousand'). Compare Jdg 5:8; 2 Samuel 10:18. These were all the young fighting men of these tribes, fit and ready to serve Yahweh. The older men and the younger remained to assist their families to settle.