"Then came Amalek"
Rabbi Yosi opens with "Blessed are you that sow beside all waters, that let the feet of the ox and the donkey range freely." He says that the Holy One, blessed be He has a tree, Zeir Anpin, that contains food for all. We are reminded that Yisrael is the trunk, and that the seventy branches are the ministers of the heathen nations. When the children of Yisrael encamped beside the water, they dominated the waters that were under the branches of that tree. The ox and donkey in the opening verse are from the left, and are brought into the discussion to emphasize that people must not give place to evil species, must remain separated from them.
Rabbi Yehuda says that "Amalek was the first of the nations" because he did not fear Elohim. He is sentenced to everlasting perdition, and his name shall be utterly blotted out from the remembrance of man. Rabbi Aba now talks about "There is a sore evil which I have seen under the sun." Rabbi Shimon explains that "sore evil" is a lack of charity, wherein people cling to their money and even bear false witness in order to gain money. The Holy One, blessed be He, had given Yisrael everything, yet they dealt with Him with false charges, and "Then came Amalek." The war of Amalek was a war above and a war below, ultimately a war against God. Amalek came to provoke Judgment over Mercy. During the war, when Moses held up his hands, he was fighting the battle above, while Joshua was fighting the battle below. From that time until the Redemption there will never be a greater battle. Joshua was worthy of fighting against Amalek because Moses saw that he came from the level of Metatron. Moses' hands were held up in faith, and it was this that gained them assistance from above. At the end, Moses built an altar called "Hashem is my banner;" Jacob built an altar and called it "El, the Elohim of the children of Yisrael."