"But to the Reubenites, the Gadites and the half-tribe of Manasseh, Joshua said, “Remember the command that Moses the servant of the Lord gave you after he said, ‘The Lord your God will give you rest by giving you this land.’ Your wives, your children and your livestock may stay in the land that Moses gave you east of the Jordan, but all your fighting men, ready for battle, must cross over ahead of your fellow Israelites. You are to help them until the Lord gives them rest, as he has done for you, and until they too have taken possession of the land the Lord your God is giving them. After that, you may go back and occupy your own land, which Moses the servant of the Lord gave you east of the Jordan toward the sunrise.” – Joshua 1:12-15 (NIV)
This passage is complicated. Looking for the lesson/message God is really giving behind it. The message I get is : God will give you peace and provide you a place of rest, but having given this to you -you are now asked to go out and assure others find it as well. Once everyone has their peace, you may rest fully in yours. It's an act of Grace. Having received grace from God you are then compelled to give Grace to others.
This passage also shows that man often takes land and fights for the sole purpose of find some semblance of peace, prosperity, and home.