Exodus 17:8

And Amalek came from the land of the south and leaped on that night a thousand and six hundred miles; and on account of the disagreement which had been between Esau and Jakob, he came and waged war with Israel in Rephidim, and took and killed (some of the) men of the house of Dan; for the cloud did not embrace them, because of the strange worship that was among them.

ויבא עמלק, at the sound of the people’s being frustrated with thirst. This is what Moses meant when he recalled the incident 40 years later in Deuteronomy 25,18 describing the Israelites’ state of mind at the time as ואתה עיף ויגע, “you were tired and worn out.” We find the expression עיף linked to water also in Job 22,7 לא מים עייף תשקה, “You do not give the thirsty water to drink.” (Eliphaz accusing Job) We also find the word applied to earth itself in Isaiah 32,2 בארץ עיפה, “in a languishing land.”

ויבוא עמלק, Amalek came;” it is difficult to understand why Amalek, if he was keen on attacking the Israelites, had waited until after they had been so miraculously saved from the Egyptians. He would have had a much easier target when the seventy people making up Yaakov’s family were on their way to Egypt. The answer may be that when G–d said to Avraham (Genesis 15,13) ועבדום וענו אותם, “they will serve them and they (their masters) will abuse them,” that as soon as Avraham would die this prophecy would be come applicable to Yitzchok and upon Yitzchok’s death it would devolve on Yaakov and his children. Amalek may have reasoned that if he were to wipe out Yaakov and family, this curse would devolve upon him. He therefore waited until the 400 years were up according to his calculation, before attacking the descendants of Avraham.