Who were these Amorites, and what gives Israel the right to their land?
The Amorites, and specifically Sihon's kingdom, represent a complex situation regarding the Promised Land.
A Canaanite People
Scripture identifies the Amorites as a major Canaanite group (Genesis 10:16). While often listed among other Canaanite tribes, they also sometimes stand as a representative term for all the inhabitants of Canaan whom God judged.
Recent Occupiers
Scholarly context suggests Sihon and his people were not original inhabitants of the land north of the Arnon River. It appears they had recently conquered this territory from the Moabites. They were, in essence, intruders who had taken possession.
Divine Transfer of Property
This is critical: the territory Sihon occupied wasn't explicitly part of the original boundaries of the Promised Land given to Abraham. However, because the Amorites had taken possession of it through conquest, and because they were a people facing God's judgment, the land they occupied was destined to pass to Israel. God's promise included not just the land of the original inhabitants but also lands unjustly seized by those whom God would displace.