After 40 years of wandering in the desert, the Israelites were ready to invade Canaan.
The sin of the Canaanites was well and truly past its due date. Their opportunities for repentance were now very limited and quite possibly restricted to running away.
Numbers 33:51-52 When you cross the Jordan River into the land of Canaan, you must drive out all the people living there. You must destroy all their carved and molten images and demolish all their pagan shrines.
The consequences of not succeeding would be dire:
Numbers 33:55-56 But if you fail to drive out the people who live in the land, those who remain will be like splinters in your eyes and thorns in your sides. They will harass you in the land where you live. And I will do to you what I had planned to do to them.
Three tribes (Gad, Reuben and half of Manassah) offered only their fighting men (translations vary: it could have been ALL fighting men or just the BEST fighting men) to help with the invasion, preferring to settle in Gilead instead. Only the other 9.5 tribes were to be allocated a share of the land promised to Abraham centuries earlier.