Adam Clarke on Exodus 17:8

Then came Amalek, and fought with Israel - The Amalekites seem to have attacked the Israelites in the same way and through the same motives that the wandering Arabs attack the caravans which annually pass through the same desert. It does not appear that the Israelites gave them any kind of provocation, they seem to have attacked them merely through the hopes of plunder. The Amalekites were the posterity of Amalek, one of the dukes of Eliphaz, the son of Esau, and consequently Israel's brother, Gen 36:15, Gen 36:16.

Fought with Israel - In the most treacherous and dastardly manner; for they came at the rear of the camp, smote the hindmost of the people, even all that were feeble behind, when they were faint and weary; see Deu 25:18. The baggage, no doubt, was the object of their avarice; but finding the women, children, aged and infirm persons, behind with the baggage, they smote them and took away their spoils.