An excerpt from Five Addresses by Rabbi Joseph B. Soloveichik, Lecture Three, Part 3:
We are duty bound to give thanks to God for this chane of name, from “all who hear will laugh at me” to “contntion” and “hatred”; for the changed attitude towards us, from laughter to hatred and fearful aggresiveness…
Moses our teacher said in his song: The peoples have heard, they tremble; pangs have taken hold on the inhabitants of Philistia. Then were the chiefs of Edom affrighted; the mighty men of Moab, trembling taketh hold upon them (Ex. 15, 14-15). At first sight it is difficult to understand why Moses was so happy about the fear that had fallen upon Edom and Moab, for Israel had no intention of attacking these two peoples. God strictly warned Moses, our teacher, concerning them: “And command thou the people, saying: Ye are to pass through the border of your brethren the children of Esau, that dwell in Seir; and they will be afraid of you; take ye good heed unto yourselves therefore; contend not with them; for I will not give you of their land, no, not so much as for the sole of the foot to tread on” Deut. 2, 4-5). Moreover, “And the Lord said unto me: Be no at enmity with Moab, neither contend with them in battle; for I will not give thee his land for a possession” (ib. 9).
The answer is that sometimes one should utter song for the fear that falls upon our enemies, even though it is an empty fear, because fear is a sign of respect and honor and openly shows that enemies recognize Am Yisrael as a living and dynamic entity. The fear itself had no practical value in Moses’ eyes, but it presaged a psychological transformation of the status of the Children of Israel in the eyes of the peoples.
I disagree with much else in the above book, especially the awful authoritarianism, and also with the larger point Rabbi J.B. Soloveichik is trying to make with the above homily, but the lesson is correct.
(I can’t copy the whole thing, so better see inside if you can.)