Numbers 32:28

"So Moses gave charge concerning them to Eleazar the priest, and to Joshua the son of Nun, and to the heads of the fathers' houses of the tribes of the children of Israel. And Moses said unto them, If the children of Gad and the children of Reuben will pass with you over the Jordan, every man that is armed to battle, before Jehovah, and the land shall be subdued before you; then ye shall give them the land of Gilead for a possession: but if they will not pass over with you armed, they shall have possessions among you in the land of Canaan. And the children of Gad and the children of Reuben answered, saying, As Jehovah hath said unto thy servants, so will we do. We will pass over armed before Jehovah into the land of Canaan, and the possession of our inheritance shall remain with us beyond the Jordan."

This paragraph recounts the public proclamation of this important decision and its solemn ratification by the tribes concerned. They no doubt meant it in good faith, but they never did what they promised to do.