Charles Ellicott Commentary Joshua 4:13

"about forty thousand ready armed for war passed over before Jehovah unto battle, to the plains of Jericho." — Joshua 4:13 (ASV)

About forty thousand. The totals of these three tribes at the last census (Numbers 26) were: Reuben (Numbers 26:7), 43,730; Gad (Numbers 26:18), 40,500; and Manasseh (Numbers 26:34), 52,700 (or for the exact half, 26,350). Thus, the entire force of the two and a half tribes might amount to 110,580. They therefore left more than half their number to protect their families and their dwellings. This does not seem inconsistent with the spirit of their agreement with Moses, or with the interpretation of that agreement by Joshua and their fellow Israelites. (Numbers 32:24; Numbers 32:26.) The permission to build cities implies the right to fortify and defend them.

Reuben, Gad, and Simeon formed the second division on the march in the wilderness (Numbers 10:18–20). Why Reuben and Gad discarded Simeon and associated themselves with part of Manasseh is not explained. (See Names on the Gates of Pearl.—Simeon.)