וַיַּךְ־צָרָ֥יו אָח֑וֹר חֶרְפַּ֥ת ע֝וֹלָ֗ם נָ֣תַן לָֽמוֹ׃
그 대적들을 쳐 물리쳐서 길이 욕되게 하시고
Mekhilta DeRabbi Yishmael
When Israel do His will, there is no sleep before Him, viz. (Psalms 121:9) "He does not slumber and He does not sleep." And when they do not do His will, there is, as it were, "sleep" before Him, viz. (Ibid. 78:66) "Then the L–rd woke as a sleeper, as a warrior rousing himself from wine."
Mekhilta DeRabbi Yishmael
And (this obtains) not with Egypt alone, but with all who afflict Israel throughout the generations. As it is written (Psalms 78:66) "And He beat back His foes. Eternal disgrace did He inflict upon them, viz. (Psalms 81:15-16) "In a moment I would humble their foes … and their time (of punishment) will be eternal", and (Isaiah 14:25) "to break Ashur in My land, etc." And thus, throughout the generations, viz. (Ibid. 26) "this is the counsel for all of the earth, and this is the hand stretched forth against all the nations" (that afflict Israel). Why so? (Ibid. 27) "For the L–rd of hosts has counseled, and who will annul it? And His hand is stretched forth, and who will turn it back?" Not Egypt ("Mitzrayim") alone, then, (is intended), but all who afflict ("meitzarim") them, throughout the generations. Thus — "for the L–rd wars for them against Mitzrayim (- "meitzarim").
Midrash Tehillim
"And he struck his enemies in the rear." Rabbi Yitzchak bar Mariyon said: "The sword of the Holy One, blessed be He, has sixteen faces, as it is stated (Ezekiel 21:21): 'Unite the right, turn to the left, wherever your face is set.' Unite [them] - four; the right - four; the left - four; and the back - four. Thus, [there are] sixteen [faces]."