Clean from suspicion[1]
ונכבשה הארץ לפני יקוק ואחר תשבו והייתם נקיים מיקוק ומישראל והיתה הארץ הזאת לכם לאחזה לפני יקוק
Once the land is conquered before Hashem, then you can return. You shall [then] be deemed innocent [in the eyes] of Hashem and the Jewish People. This land shall [then] be yours for an inheritance, before Hashem[2]
After the Jews conquered the land on the east side of the Jordan River, it became considered part of the land of Israel. The land of Israel proper, on the west side of the Jordan River, still had to be conquered. Two and a half tribes, that of Reuven, Gad, and half of Menashe, requested for the opportunity to have their portion be solely in the land on the east side of the Jordan River. Moshe took this to mean that they weren’t interested in helping their brethren conquer the rest of the land of Israel. This could lead to distrust, quarrels, and maybe even civil war. Moshe reasoned with them that they’ll be allowed to be the sole inheritors of this part of the land if they help with the war effort on the west side of the Jordan. Upon victory, they’ll be welcome to return to their families on the east side of the Jordan River, and begin to settle it.