exodus 17:8 Then came Amalek, and fought with Israel in Rephidim. - Free Bible Online

      Ex 17:8-16. ATTACK OF AMALEK.

      8. Then came Amalek--Some time probably elapsed before they were exposed to this new evil; and the presumption of there being such an interval affords the only ground on which we can satisfactorily account for the altered, the better, and former spirit that animated the people in this sudden contest. The miracles of the manna and the water from the rock had produced a deep impression and permanent conviction that God was indeed among them; and with feelings elevated by the conscious experience of the Divine Presence and aid, they remained calm, resolute, and courageous under the attack of their unexpected foe.
      fought with Israel--The language implies that no occasion had been furnished for this attack; but, as descendants of Esau, the Amalekites entertained a deep-seated grudge against them, especially as the rapid prosperity and marvellous experience of Israel showed that the blessing contained in the birthright was taking effect. It seems to have been a mean, dastardly, insidious surprise on the rear (Nu 24:20; De 25:17), and an impious defiance of God.

JFB.