This statement, in Nicias' letter from Sicily, has puzzled readers and editors, and it has baffled translators, who expect phrases with ⋯π⋯ προɸάσει to indicate a pretext or ostensible reason for an action. In my first discussion of prophasis I suggested that ‘what Nicias means is that they are leaving without offering any other reason or pretext’ (except αὐτομολία), and Dover, in the Historical Commentary, goes halfway towards accepting this interpretation (‘though it is obscure, I do not think it is impossible’); but it does not satisfy me now, and it is firmly rejected by Hunter Rawlings. A different approach to the question is needed.
Nicias is not concerned with any excuses or pretexts that these deserters are offering. He is explaining the ‘true prophasis’ of this epidemic of desertion, and does so in terms of opportunity — they can find employment in the enemy's forces and there are numerous opportunities elsewhere (Sicily is a large island). Translators may not want to render ⋯π' αὐτομολίας προɸάσει as ‘taking the opportunity to desert to the enemy’, arguing that ‘prophasis cannot mean opportunity’. But a prophasis is an explanation, true or false, that can be given for an action, and the best explanation (or excuse) can often be found in the circumstances — the opportunity that tempts or encourages. These deserters have a prophasis.
This is not the only passage in Thucydides where ‘opportunity’ seems to be the word that translators need. A good example is in 4.126.5, where Brasidas is explaining how barbarian armies fight: αὐτοκράτωρ δ⋯ μάχη μάλιστ' ἃν κα⋯ πρόɸασιν το⋯ σᾡζεσθαί τινι πρεπόντως πορίσειε.