فَجَعَلَهُمْ كَعَصْفٍ مَّأْكُولٍ
The fāʾ at the head conveys "immediately after" and "as a result of this". No process, no struggle, no counter-move is narrated in between. The stones were thrown — the outcome.
There is no battle scene at all in the sūra. The army does not resist, does not flee, does not fight. This is a verdict the narration establishes by its own manner: the other side found no opportunity to be the subject of the event.
This verb had occurred in the second verse as well: "He placed their scheme inside a bringing-to-nothing". The same verb is used at two different points of the sūra, and both convey a transformation. The plan was put into a state; then those who made the plan were put into a state. The same hand does both.
Root ʿ-ṣ-f. What the word describes: the part of the crop left over apart from the grain — stalk, leaf, husk, chaff. The dry residue left in the field after the harvest and scattered about by the wind.
From the same root comes ʿāṣif: a violent wind that sweeps things away. This word occurs in the description "a violent wind" (Yūnus 10/22). The root carries the idea of being scattered within it.
Its clearest occurrence in the Qurʾān is in Sūrat al-Raḥmān: "the grain with its stalks and leaves, and the fragrant plants" (al-Raḥmān 55/12). There ʿaṣf is the worthless part beside the grain. Even where it is being counted as a favour, the grain and the stalk are kept apart.
This is the first layer of the comparison: an army — with its elephants, its cavalry, its imperial backing — is likened to the dry stalk left in a field. Not the grain, which has an inside, but the husk, which has only an outside.
The construction here brings the comparison down one step further: not merely "like straw", but "like eaten straw". The classical exegetes are found to explain this expression in two ways:
The first: fodder that an animal has eaten, digested and passed out. That is, a residue chewed once more, worked over, now good for nothing at all. On this reading the sūra would be describing the scattered state of the bodies.
The second: a crop eaten out from within by worm or insect. Seen from outside the ear still stands and looks whole; but it has been emptied inside. Touched by the hand, it falls apart.
The second reading sits more sharply on the situation the event describes, and accords better with the sūra as a whole. For what the sūra has been working on from the beginning is the difference between apparent size and real durability. The elephant looks large; the army looks imposing; the ear of grain stands upright. In all three there is a mismatch between the outward appearance and the emptiness within.
I offer this not as a preference adopted but as a strong reading; neither interpretation runs against the text.
Look at the rhyme of the five verses: fīl — taḍlīl — abābīl — sijjīl — maʾkūl. Four end with a long ī, the last with ū. A long vowel stretches the sound out and weights its close; the vowel of the last verse changes and so brings the reader to a stop. In short sūras the sound is a carrier of the meaning; so it is here.