וְחִזַּקְתִּ֣י אֶת־לֵב־פַּרְעֹה֮ וְרָדַ֣ף אַחֲרֵיהֶם֒ וְאִכָּבְדָ֤ה בְּפַרְעֹה֙ וּבְכָל־חֵיל֔וֹ וְיָדְע֥וּ מִצְרַ֖יִם כִּֽי־אֲנִ֣י יְהוָ֑ה וַיַּֽעֲשׂוּ־כֵֽן׃
И я ожесточу фараона'сердце, и он последует за ними; и Я воздаю Мне честь за фараона и за все его войско; и узнают Египтяне, что Я Господь.' И они так и сделали.
Rashi
I will now sing for my beloved The prophet says: “I will now sing for my beloved and in his place and as his messenger. [The word לִידִידִי would usually mean, ‘to my beloved.’] Similar to (Exodus 14: 4): “The Lord will wage war for you (לָכֶם), for you [not ‘to you’].”
Sforno
ולא ישמע אליכם פרעה; not before the plague materialises after the warning, nor even after the plague did come to pass. He will not listen to you even after having endured many such plagues. Seeing that this is so, I will be forced to bring retribution upon them. This retribution will take the form of the killing of the firstborn as well as the drowning of all the military might of Egypt in the sea. The retribution will be seen as a punishment to fit their crimes. All the other plagues are only designed to encourage repentance, not as a form of retribution. Four times the Torah introduces this objective of the plagues. In 7,17 the Torah writes למען תדע כי אני ה', “so that you will know that I am the Lord.” In 8,18 the Torah writes: בעבור תדע כי אני ה' בקרב הארץ, “so that you will know that I am the Lord in the midst of the earth (not only in heaven).” In 9,29 the Torah writes: למען שיתי אותותי אלה בקרבו ולמען תספר...וידעתם, “this is why I perform all these miracles of Mine right in its midst, so that you will tell…. and finally realise that there is no one like Me anywhere (9,14).”
Or HaChaim
אשר ראיתם את מצרים, "the way you have seen Egypt, etc." G'd explained that the reason He had allowed the Egyptians to assume such a threatening posture was only because they would never again assume a threatening posture such as this. The Egyptians would soon collapse in spite of their fear-inspiring guardian angel.