Why did Amalek attack Yisrael at this juncture? What had Yisrael done wrong?
Rashi: Hashem is always close to You, to help you in time of need, and you turn round and ask "ha'Yesh Hashem b'Kirbeinu Im Ayin"?! By your life, the dog will come and bite you and you will cry out to Me. Then you will know where I am". 1
Seforno: He came because there was strife, and he took advantage of the fact that they were thirsty.
Targum Yonasan: He attacked the B'nei Dan, whom the Cloud had expelled from the camp on account of the Avodah Zarah that they were carrying with them. 2
Mechilta: Because they slackened from learning Torah. 3
Rashi: This can be compared to a father traveling along the road with his son on his shoulders (corresponding to "Al Kanfei Neharim"
Why did Amalek not attack Yisrael on their way down to Egypt, when they numbered only seventy souls?
Da'as Zekeinim: Because he realized that, once Yitzchak died, the fulfillment of the prophecy "va'Avadum ve'Inu osam Arba Me'os Shanah" would fall upon Yakov, Esav and their children. If he were to kill Yakov and his descendents, it would fall upon him [to a larger extent] to fulfill. So he waited until Ya'akov's children had 'paid the debt' before attacking. 1
Yisrael were surrounded by Anenei Kavod. How could Amalek attack?
Mechilta, Shemos Rabah 26:3: Amalek entered under the corners of the clouds 1 , and kidnapped Yisraelim and killed them