Who are the messengers?
Rashi: They are many camps of troops going on My mission "to terrify Kush."
Radak: It is an army of Goyim; it is as if they go on Hashem's mission for He decreed against [Kush].
Malbim: This is via His Hashgachah. Not only will those who helped Egypt in the war be broken, and not only those who are on the side of Migdol Sevenah - rather, also Kush which is on the other side, like it says "mi'Migdol Sevenah v'Ad Gevul Kush" (29:10).
What are "ba'Tzim"?
Rashi: This is like "v'Tzim 1 mi'Yad Kitim" (Bamidbar 24:24). Also Yonason translates 'legions.'
Radak: They are boats, like "v'Tzi 2 Adar Lo Ya'avrenu" (Yeshayah 33:21). A big river surrounds Kush; one can enter only via boats - "v'Hu Sovev Es Kol Eretz Kush" (Bereishis 2:13). And so it says "me'Ever l'Naharei Kush; ha'Shole'ach ba'Yam Tzirim... Al Pnei ha'Mayim" (Yeshayah 18:1-2).
What is the meaning of "Lehacharid Es Ksuh Betach"?
Rashi: To terrify Kush, which dwells confidently 1 . Radak - the nation that will come upon her suddenly will terrify her.
Malbim: They were confident because (a) the enemy was far from them, and (b) there was water in between - me'Ever l'Naharei Kush" (Yeshayah 18:1).
What is "k'Yom Egypt"?
Radak: Like there was Chalchalah (trembling) in Kush at the time of Egypt's Churban (4). Now they will tremble over their own [affliction]!
What came?
Radak: The time of their destruction.
Malbim: The trembling - this is not a distant report, and not about the future!