Commentary for Deuteronomy 4:46

אשר הכה משה, whom Moses and Israel had defeated, etc. How could the Torah describe this as having occurred "when the Israelites left Egypt," when we all know that this war occurred in the fortieth year of the Israelites' wanderings? Perhaps all the Torah wanted to say was that ever since the Exodus of the Israelites from Egypt it had been one of the objectives to defeat these two kings. In fact, had it not been for the debacle with the spies, the Israelites would have defeated these two kings in the second year after the Exodus.

Furthermore, the Torah may hint that the terror of the Israelites defeating them had been part of the lives of the kings of these two countries and their people. For every practical purpose then these kings and their countries could have been considered as defeated already at that time (compare Bereshit Rabbah 80 that these kings had a tradition they would fall victim to the Israelites). The fact that the Torah speaks about the Israelites inheriting these lands only in the next verse instead of at the end of verse 46 indicates that our interpretation is correct.