Or HaChaim on Numbers 32:23/אור החיים על במדבר ל״ב:כ״ג

1 ואם לא חעשון כן הנה חטאתם, "But if you will not do so, behold you have sinned against the Lord, etc." This appears problematical. We would have expected Moses to say only that if these two tribes did not keep the bargain they would not be given the lands on the East Bank. However, the meaning of these words is that if the two tribes would not accept Moses' command to treat the conquest of the West Bank as a מלחמת חובה, an obligatory war fought in order to take revenge on the enemies of G'd, they would have sinned even if they did cross the Jordan fully armed. Moses spoke of that sin in the past tense, i.e. "your sin would commence already at this time."

2ודעו חטאתכם אשר תמצא אתכם. "and be aware that your sins will find you out." Moses warned that at the appropriate time when they would find themselves in imminent danger G'd would exact retribution from the guilty, i.e. they would fall in battle. Remember what Tur Orach Chayim chapter 54 writes that in a war of expansion soldiers are sent home if they have been guilty only of speaking needlessly between such parts of the morning prayers as ישתבח and יוצר אור. Moses warned now that all the promises he had made concerning these soldiers returning home unharmed would be null and void unless they kept to all the details of his conditions. If they failed to do this they would not be free either in the eyes of G'd or the people. The very fact that they had sustained casualties would be proof of their being guilty. Compare what I have written on the words עבדיך יעשו כאשר אדני מצוה in verse 25.