The previous word: it is the delay of their punishment until the Hereafter. He says: Were it not for this delay, the destruction of us like that of 'Aad and Thamud would have been inevitable for these disbelievers. And the inevitability: it is either a source that is necessary described by it, or it is a form meaning that which causes necessity, as if it is a tool for necessity due to its extreme necessity, as they said: a persistent adversary. And a specified term cannot be without being attached to a word or to the pronoun in 'it would have been', meaning that the swift punishment and the specified term are both necessary for them, just as they were necessary for 'Aad and Thamud, and the specified term is not independent of the swift punishment.