אֵיכָ֞ה יִרְדֹּ֤ף אֶחָד֙ אֶ֔לֶף וּשְׁנַ֖יִם יָנִ֣יסוּ רְבָבָ֑ה אִם־לֹא֙ כִּי־צוּרָ֣ם מְכָרָ֔ם וַֽיהוָ֖ה הִסְגִּירָֽם׃
How should one chase a thousand, And two put ten thousand to flight, Except their Rock had given them over And the LORD had delivered them up?
Rashi on Deuteronomy
איכה ירדף אחד HOW ONE MAN of the nations COULD PURSUE אלף A THOUSAND of the Israelites.
Sforno on Deuteronomy
איכה ירדוף אחד אלף, they should have realised that if they were able to pursue the Israelites, one against 1000, that this was not something natural.
Or HaChaim on Deuteronomy
איכה ירדף אחד אלף, "How could one man pursue a thousand?" This refers to the fact that one Gentile suddenly was able to put to flight one thousand Jews. During all the normal wars between the various nations such a thing was unheard of. Not a single enemy amongst the nations possesses such superiority over his opponents.