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The collective noun jund, which primarily denotes "a host" or "an army", has also the meaning of "created beings", in this context obviously human beings; in combination with the particle ma, "any number of human beings". The term hizb (of which ahzab is the plural), on the other hand, denotes "a party" or "a group of people of the same mind" or "people leagued together", i.e., for a definite purpose.
This verse alludes to the defeat of the Meccan pagans later at Badr.
Of course they cannot frustrate Allah's Purpose. In that world-they will be ignominiously routed, even if they form the strongest confederacy of the Powers of Evil that ever could combine. Cf. the last clause of verse 13 below.