Remember the Peor incident, where the men of Israel prostituted themselves to Baal along with the women of Moab? Turns out that was led by Balaam, the same Balaam who spoke oracles from the Lord against Moab. After defeating Midian, they begin to divide the spoils of war. When the tribes of Reuben, Gad, and the half tribe of Manassah, see that this land is good for cattle, they ask if they can settle there. Moses assumes that they are trying to avoid going to war with their brothers. By the end of the conversation, they cut a deal with Moses, assuring him that they, too, will go to war with the rest of Israel before coming back to settle in the land.