Midrasch zu Ijow 8:11

(Exodus 17:8) "And Amalek came": This verse is recondite and is explicated by (Iyyov 8:11) "Can the rush grow without swamp? Can grass grow without water?" So, Israel without Torah. And because Israel separated from Torah, therefore, the foe (Amalek) came upon them. For the foe comes only by reason of sin and transgression. Thus, "And Amalek came, etc." R. Elazar Hamodai says: "And Amalek came": Amalek "sneaked" under the edges of the cloud and snatched souls of Israel and killed them, viz. (Devarim 25:18) (Amalek) "who met you on the way … when you were faint and weary and (Amalek) did not fear G d." Others say: This refers to Israel, who (at that time) did not have mitzvoth in their hands.

Then came Amalek and fought with Israel in Rephidim (Exod. 17:8). R. Eliezer the son of Hisma said: This verse should be understood and interpreted in relation to the verse quoted in Job: Can the rush shoot up without mire? Can the reed grass grow without water? (Job 1:11). Just as that is impossible, so is it impossible for Israel to survive unless the people devote themselves to the law. It was because Israel turned away from the law that the enemy attacked them. Indeed, the enemy attacked them only as a consequence of their sins and transgressions. Hence it says: Then came Amalek.