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