'Why does the Torah add "beYad Chazakah"?
Rashi: This referred to the strength of Yitzchak's promise to Eisav "ve'ha'Yadayim Yedei Eisav!"
And what did he mean when he concluded "ve'Achariso Adei Oveid"?
Rashi: He meant that ultimately, Yisrael will destroy Amalek. 1
Targum Onkelos: 'Ultimately, they will be destroyed forever'.
Targum Yonasan: 'Ultimately, in the days of Mashi'ach, they (Amalek) will join forces with the nations of the east to wage war against Yisrael; all of them will be destroyed forever'.
Rashi: As the Torah writes in Devarim 25:19 "Timcheh es Zeicher Amalek!" Rashi in Koheles (8:9): 'I saw and I took to heart ... how a person dominated his friend and overpowered him, and how ultimately, it turned out to his detriment: Amalek overpowered Yisrael, but in the end "veAchriso Adei Oveid" - and that is what happened to Nevuchadnetzar and to Sancheriv'.