And God said, “This is the guarantee ¹  of the covenant I am making ²  with you ³  and every living creature with you, a covenant ⁴  for all subsequent ⁵  generations:

He left those nations simply because he wanted to teach the subsequent generations of Israelites, who had not experienced the earlier battles, how to conduct holy war. ¹ 

There was a famine in the land, subsequent to the earlier famine that occurred ¹  in the days of Abraham. ²  Isaac went to Abimelech king of the Philistines at Gerar.