Charles Ellicott Commentary Exodus 17:8

"Then came Amalek, and fought with Israel in Rephidim." — Exodus 17:8 (ASV)

Then came Amalek. —The Amalekites had not been previously (except in the anticipatory notice of Genesis 14:7) mentioned as a nation. Their name marks them as descendants of Amalek, the grandson of Esau (Genesis 36:12; Genesis 36:16); and it would seem that they early became the predominant people in the Sinaitic peninsula. Balaam speaks of them as the first of the nations (Numbers 24:20); and though we do not meet with the name in the Egyptian records, it is probable that they were among the hostile nations whom we find constantly contending with the Egyptians on their north-eastern frontier.

Though Edomites, they are always regarded as a distinct race, and one especially hostile to Israel (Exodus 17:16). Their present hostility was not altogether unprovoked. No doubt they regarded the Sinaitic region as their own, and as the most valuable portion of their territory, since it contained their summer and autumn pastures.

During their absence in its more northern portion, where there was pasture for their flocks after the spring rains, a swarm of emigrants had occupied some of their best lands and threatened to seize the remainder. Naturally, they would resent the occupation. They would not understand that it was only temporary. They would regard the Israelites as intruders, robbers, persons entitled to scant favor at their hands.

Accordingly, they swooped down on them without mercy, attacked their rear as they were on the march, cut off their stragglers, and slew many that were feeble, faint, and weary (Deuteronomy 25:17–18). They then encamped in their neighborhood, intending to renew the struggle on the next day. It was under these circumstances that Moses had to make his arrangements.