The conquest of the land described in the book of Yehoshua took seven years (Erchin 13a, h/t DoubleAA).1 It is only at the end of the conquest that Yehoshua dismisses the "shock troops".
We know from 1:14 that "all the mighty men of valor" from the tribes of Reuven, Gad, and 1/2 of Manashe led the conquest, and the declaration in 1:18 (calling for anybody who rebels from this command to be put to death) sounds like they took it seriously.
So if all the men capable of fighting were with Yehoshua conquering the land, who was protecting their families on the other side of the Yarden from the surrounding peoples? Or was there a miracle and the surrounding peoples did not attack this easy target during all those years?
1 Caleb was 40 when he spied the land and is 85 now (14:10), and they were in the wilderness 38 years after the spies' mission.