וְיֹֽשְׁבֵיהֶן֙ קִצְרֵי־יָ֔ד חַ֖תּוּ וַיֵּבֹ֑שׁוּ הָי֞וּ עֵ֤שֶׂב שָׂדֶה֙ וִ֣ירַק דֶּ֔שֶׁא חֲצִ֣יר גַּגּ֔וֹת וּשְׁדֵפָ֖ה לִפְנֵ֥י קָמָֽה׃
Therefore their inhabitants were of small power, They were dismayed and confounded; They were as the grass of the field, And as the green herb, As the grass on the housetops, And as corn blasted before it is grown up.
Rashi on II Kings
Their inhabitants were short of strength. Those whom I weakened before you,16I.e., it is a Divine gift for which Sancheiriv could claim no credit. and the might is not yours.17The nations that Sancheiriv conquered had already been condemned to punishment and destruction. You are not mighty, rather they are weak.18In Tehilim 129:6 the verse states” … like the grass on rooftops which even before it is plucked, withers.”
Rashi on II Kings
Like grass of the rooftops. Which dries quickly.19Alternatively, grass is unable to move and defend itself from being cut.—Ralbag