As the Israelites prepared to invade the Land of Canaan, the tribes of Reuben, Gad and part of Menashe approached Moses with a request to be allowed to stay on the East Bank where there was excellent prairie land, good for herds. Moses at first objected to splitting the people. All the tribes had been involved in conquering the territory on the East Bank of the Jordan. It would not fair if two and a half tribes now withdrew from helping invade the West. He accused them of undermining him and the people in general. But they returned and reassured Moshe that they would cross over and help conquer Canaan and remain committed to the Children of Israel in general.
When they approached Moses, they said “We will build pens for our sheep and flocks and cities to protect our children and then we join you to lead the invasion. And we will not return home until it is done.”
Moses then replied turning their words into a contractual deal. He agreed with a double condition. This double condition was stated in the formula that if you do what you say you will fine, but if you do not do what you say you will do, there will be serious consequences. This double condition was adopted in the Talmud as the model for all contractual conditions. Moses must also have upbraided them for thinking primarily of their flocks and only as an afterthought, their children. By the end of the negotiations they changed the order of priorities to put their wives and children first and their cattle second.
For all their commitment, the fact was that living detached from the main body of the people, over time, weakened the involvement of the two and a half tribes. Many of them assimilated into the other nations beyond the Jordan and the remnant was the first to be exiled.