“After the enemy destroyed the bridge that was not guarded, all the hoplites left their weapons in the plain in order to (as they claimed) keep peace now at least.”
Why is the aorist form used here? Doesn’t it make much more sense to use φυλασσομένην instead(the present participle), since the bridge needs guarding not just for one moment, but in an open and ongoing period of time?
At least at the moment when it was destroyed, it was not guarded. A present would mean that it was not being guarded, over some time period, or habitually not guarded. There is a passage in Herodotus that this is very similar too, if I recall correctly. I’ll try to look it up.
Notice that this is the transitive usage (necessarily, as it’s a passive). I don’t think that the intransitive meaning, “being on guard”, generally takes the aorist.
EDIT: The Herodotus that I was thinking of: πυνθανόμεθα γὰρ Δαρεῖον ἐντείλασθαι ὑμῖν ἑξήκοντα ἡμέρας μούνας φρουρήσαντας τὴν γέφυραν. Transitive here, with aorist φρουρήσαντας, basically the same meaning. So in my head, closer to commanded you to “make a watch alone on the bridge that was 70 days long” than it would be to “be watching the bridge alone for 70 days”. IMO, anyway. A very minor difference in mental conception.
The aorist is roughly equivalent to “which had not been guarded,” while the present would mean "which was not being guarded.”
The Herodotus quote is incomplete as it stands, and the aorist participle means “after keeping watch on the bridge for only sixty days” (μουνας with ημερας, not “alone”), while the present participle would mean “while keeping watch …”. It needs an infinitive (either present or aorist) to complete the sense.
That would require the present participle: the bridge was not being guarded at the time it was destroyed—the enemy destroyed it while it was not being guarded.
I just posted a very similar query and subsequently found this explanation in “Complete Ancient Greek”, which may be helpful:
The difference here between the present and aorist participle is one of aspect, not time. A present participle indicates a condition or process (first and second examples); an aorist participle indicates an event (third and fourth examples):
1)πάντας λανθάνει δάκρυα λείβων. He sheds tears without anyone knowing (lit. he [in] shedding tears escapes the notice of all). 2) Mενέλεως ἡμᾶς ἔλαθε παρών. Menelaus was present without us knowing (lit. Menelaus [in] being present escaped the notice of us).
3) ἔϕθασαν τὸν χειμῶνα ἀνασπάσαντες τς ναῦς. They hauled up their ships before winter (lit. they [in] hauling up their ships anticipated the winter).
4) ἔϕθασαν οἱ Σκύθαι τοὺς Πέρσᾱς ἐπὶ τὴν γέϕῡραν ἀϕικόμενοι. The Scythians arrived at the bridge before the Persians (lit. The Scythians [in] arriving at the bridge anticipated the Persians).
In sentences of this type it is, in fact, immaterial whether λανθάνω /ϕθάνω appear as the finite verb with an accompanying participle (as above), or as the participle (always aorist; the participle here has no temporal force) with an accompanying finite verb. Thus in the first and third examples we could have, with identical meanings: πάντας λαθὼν δάκρυα λείβει (lit. escaping the notice of all, he sheds tears); ϕθάσαντες οἱ Σκύθαι τοὺς Πέρσᾱς ἐπὶ τὴν γέϕῡραν ἀϕκοντο (lit. the Scythians, anticipating the Persians, arrived at the bridge).
Betts, Gavin; Henry, Alan. Complete Ancient Greek: A Comprehensive Guide to Reading and Understanding Ancient Greek, with Original Texts (Complete Language Courses) (p. 254). John Murray Press. Kindle Edition.