Where did Amalek come from?
Targum Yonasan: He traveled sixteen hundred Mil (four hundred Parsah) from Har Se'ir in the south to fight with Yisrael.
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 be'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 were lax in Torah-study. 3
Oanayim la'Torah (on Pasuk 9): He came to stop Yisrael from receiving the Torah on Har Sinai.
Rashi: This can be compared to a father traveling along the road with his son on his shoulders (corresponding to "Al Kanfei Neharim"
Why does th Torah call the place 'Refidim' and not by its new name 'Masah u'Meriah'?
Oznayhim la'Torah: Now that Amalek had attacked Yisrael (because they were lax in Torah), its harsh name 'Masah u'Merivah' is scrapped once and for all, and, from now on, it is known (exclusively) by its original (less harsh) name 'Refidim'.
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 Ya'akov, Eisav and their children. Consequently, if he were to kill Ya'akov and his descendents, it would fall largely upon him. So he waited until Ya'akov's children had 'paid the debt' before attacking. 1
Yisrael were surrounded by Ananei Kavod. How could Amalek attack them?
Mechilta, Shemos Rabah 26:3: Amalek entered under the corners of the clouds 1 , and kidnapped Yisraelim and killed them