We must do as the LORD commands.

Now the children of Reuben and the children of Gad had a very great multitude of livestock; and when they saw the land of Jazer and the land of Gilead, that indeed the region was a place for livestock, Therefore they said, “If we have found favor in your sight, let this land be given to your servants as a possession. Do not take us over the Jordan.” And Moses said to the children of Gad and to the children of Reuben: “Shall your brethren go to war while you sit here? We will not return to our homes until every one of the children of Israel has received his inheritance. For we will not inherit with them on the other side of the Jordan and beyond, because our inheritance has fallen to us on this eastern side of the Jordan.” Then Moses said to them: “If you do this thing, if you arm yourselves before the Lord for the war, and all your armed men cross over the Jordan before the Lord until He has driven out His enemies from before Him, and the land is subdued before the Lord, then afterward you may return and be blameless before the Lord and before Israel; and this land shall be your possession before the Lord . But if you do not do so, then take note, you have sinned against the Lord; and be sure your sin will find you out. Build cities for your little ones and folds for your sheep, and do what has proceeded out of your mouth.” And the children of Gad and the children of Reuben spoke to Moses, saying: “Your servants will do as my lord commands.
Numbers 32:1,5-6,18-25 - NKJV