The phrase "until Dan" appears not only in Moses' vision but much earlier in the Torah, when Abraham "pursued them until Dan" (Genesis 14:14) during his rescue of his nephew Lot. The Mekhilta raises the same problem: the tribes had not yet come to the land, and the territory had not yet been divided. There was no place called Dan. So what does the verse mean?
The answer takes a darker turn than the parallel teaching about Moses. When Abraham arrived at the place that would one day be called Dan, God revealed to him what would happen there in the distant future: "In this place, your children are destined to serve idolatry." Abraham was shown the sin of the golden calves that Jeroboam would later set up in Dan (1 Kings 12:29), one of the great apostasies of Israelite history.
And when Abraham saw this — when he glimpsed his own descendants worshipping idols on this very ground — "his strength waned." The mighty warrior who had just defeated four kings with a handful of men suddenly lost his power. Not because of any physical enemy, but because of a vision of spiritual failure centuries in the future.
The Mekhilta presents Abraham's military strength as directly connected to his spiritual confidence. As long as he believed his descendants would remain faithful, he was unstoppable. The moment he saw their future betrayal, his body gave out. The geography of "Dan" carried not just a place name but a prophecy of sin — and that prophecy was heavy enough to break the patriarch mid-stride.