To say that the Amorites had a border is to say that they were so strong that they had a defined territory that they could call their own. They were not just hiding out in Israelite territory; they had their own land, and their own recognized border!
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In Deuteronomy 7:1–26, the Lord commands his people to devote the Canaanites to complete destruction. Israel's failure to do so is a failure to keep the Lord's covenant. The issue was not that they were unable to drive out the enemy; rather, the issue was that they were more willing to cohabitate with them.
36 And the border of the Amorites ran from the ascent of Akrabbim, from Sela and upward.