Exodus 17:8 - Albert Barnes' Notes on the Bible

Then came Amalek - The attack occurred about two months after the Exodus, toward the end of May or early in June, when the Bedouins leave the lower plains in order to find pasture for their flocks on the cooler heights. The approach of the Israelites to Sinai would of course attract notice, and no cause of warfare is more common than a dispute for the right of pasturage. The Amalekites were at that time the most powerful race in the Peninsula; here they took their position as the chief of the pagans. They were also the first among the pagans who attacked God’s people, and as such were marked out for punishment (see the marginal references).