The Ginzei HaMelech writes that the word es implies that the Jews were allowed to annihilate these enemies, despite the fact that Amalek is the only nation we are allowed to annihilate. This es includes nations not of Amalek involved in the attempted annihilation of the Persian Jews.
The Ginzei HaMelech continues by questioning how we can annihilate another nation. He quotes the Rambam (Mishneh Torah, Hilchos Melachim 5:4-5) who points out that the Assyrian king, Sancherev, mixed the nations that he conquered, and we no longer know the actual national pedigree of any people. He answers by quoting his friend, R’ Akiva Stolper that this permission includes any nation that exhibits the characteristics of Amalek. He proves this with a story of R’ Chaim of Volozhin, who once visited St. Petersburg. He saw a little boy there named Nikolai, about whom he confessed to his companion, “he worries me. He is Amalek.” That boy grew up to be the raging Jew hater, Czar Nikolai. Nikolai’s pedigree to the Russian royal throne was unquestionable, so the only way for him to be Amalek is in his personality.
Here, too, these people had the characteristics of Amalek. Despite the obvious fact that the Jews were ascending, these people were still planning an attack! Only Amalek would do something like that, as they did when they attacked the victorious Jews leaving Mitzrayim (Shemos 17:8) so many years before. The Midrash (Tanchuma, Ki Teitzei 9) compares Amalek to a person who knowingly scalds oneself by jumping into a boiling hot pool in order to convince others to do it, too.