Quote from Dr. Dan
What if it was not land but just peace and cooperation? What if it was something like today's Jordan (which is technically biblical Israel) and Israel had not conquered any land nor was there an Exchange for land in a deal...just peace? Are we permitted to do such things?
In the last year of Moshe's life, the nation of Israel conquered the territory which would later become the tribal portion of Reuven, Gad, and half of Menashe.
When it sounded like the tribes of Reuven and Gad were suggesting, that we be content with the part of Israel we captured and do no more to get all the territory that we are supposed to get, Moshe accused those tribes of committing a crime similar to the crime of the spies. See Bamidbar/Numbers chapter 32 for details.
Moshe was upset with the attitude of the tribes of Reuven and Gad, even though they were not giving up on anything they already conquered and even though the plan would have "bought short term peace" with the 7 nations of Canaan, who were temporarily too frightened to start up with the nation of Israel.
In the long run, Moshe knew that leaving the 7 nations of Canaan in any part of Israel would ultimately lead to the security problems mentioned in Bamidbar/Numbers 33 verses 55 and 56.
And in case, it was against G-d's will to leave those Gentiles that deny the truth of Torah in the land of Biblical Israel.