Reuven and Gad are to Fight and Conquer
Moshe relates how G-d gave Gilead to Machir and He gave the region from Gilead to the brook of Arnon (including the entire brook) and the brook of Yabbok (which is the border of the Ammonite children) to the tribes of Reuven and Gad.
The inheritance of the children of Gad was on the eastern side of the Jordan, meaning some of the Jordanian land fell within their lot,, including the region up until the city of Kinneret (which was within the portion of the tribe of Naftali) which was on the western side of the Jordan.
And at that time, G-d commanded the sons of Reuven and Gad to go ahead of the rest of the Israelites to fight, since they were mighty, and conquer the land.
However they were to leave their families and animals behind in the cities G-d gave them, and only return to them after the land is conquered.
G-d also commanded Yehoshua at that time not to fear the fighting because just as Yehoshua saw how G-d had made the kingdoms of Sichon and Og fall, so would G-d have the other kingdoms which they would pass through fall and it is He who is fighting.
Chumash "On the Go" 