(The view expressed in this blog are my own and should not be taken as inspired in any way.)
Joshua 12:1–6, “Now these are the kings of the land, which the children of Israel smote, and possessed their land on the other side Jordan toward the rising of the sun, from the river Arnon unto mount Hermon, and all the plain on the east: Sihon king of the Amorites, who dwelt in Heshbon, and ruled from Aroer, which is upon the bank of the river Arnon, and from the middle of the river, and from half Gilead, even unto the river Jabbok, which is the border of the children of Ammon; And from the plain to the sea of Chinneroth on the east, and unto the sea of the plain, even the salt sea on the east, the way to Bethjeshimoth; and from the south, under Ashdothpisgah: And the coast of Og king of Bashan, which was of the remnant of the giants, that dwelt at Ashtaroth and at Edrei, And reigned in mount Hermon, and in Salcah, and in all Bashan, unto the border of the Geshurites and the Maachathites, and half Gilead, the border of Sihon king of Heshbon. Them did Moses the servant of the LORD and the children of Israel smite: and Moses the servant of the LORD gave it for a possession unto the Reubenites, and the Gadites, and the half tribe of Manasseh.”
When Israel was preparing to enter the Promised Land they encountered warlike nations on the eastern side of the River Jordan. Even though Israel intended to cross over the Jordan to take possession of the Promised Land these people arbitrarily chose to engage Israel in battle and were soundly defeated. The mountains on the eastern side of the River Jordan are higher than the mountains on the western side of the River so they were fertile lands that were good for sheep and cattle. Some of the tribes decided that they wanted to stay on that side of the river because they had large herds and flocks. Many years later they paid a price for this choice. Later, after the kingdom was divided, Israel went into exile in two separate conquests (see 1 Chron 5:26, “And the God of Israel stirred up the spirit of Pul king of Assyria, and the spirit of Tilgathpilneser king of Assyria, and he carried them away, even the Reubenites, and the Gadites, and the half tribe of Manasseh, and brought them unto Halah, and Habor, and Hara, and to the river Gozan, unto this day.”) These people, who chose land outside the Promised Land did not receive the some protection as those who chose to come into the Promised Land.