In Numbers 32, we read of the tribes of Reuben and Gad making a request of Moses to be allowed to stay with all their livestock and not go across the Jordan with the rest of the people. But Moses says to them:
“Shall your brothers go to the war while you sit here? Why will you discourage the heart of the people of Israel from going over into the land that the Lord has given them? Your fathers did this, when I sent them from Kadesh-barnea to see the land. For when they went up to the Valley of Eshcol and saw the land, they discouraged the heart of the people of Israel from going into the land that the Lord had given them.
All of Israel was to participate in the conquest of the Promised Land. If Reuben and Gad didn’t join in they would discourage them all. Moses reminded them how God’s anger was kindled against the people when they were too discouraged to enter 40 years prior. They assured Moses that they would join in the battles, although their wives, children and livestock would stay in the land. But Moses warned them:
“If you will do this, if you will take up arms to go before the Lord for the war, and every armed man of you will pass 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 after that you shall return and be free of obligation to the Lord and to Israel, and this land shall be your possession before the Lord. But if you will not do so, behold, you have sinned against the Lord, and be sure your sin will find you out.