δέδια, μὴ λελήθασιν ἡμᾶς οἱ πολέμιοι περιστάντες
I am afraid that our enemies are surrounding us unawares.
A subordinate clause in the indicative (usually the perfect indicative) without ἄν, introduced by μή, signals a causative object with verbs of fearing when the object of fear is in the past and thus can no longer be changed.
In general, a large variety of verbal forms may be used after historical tenses, all of which are explicable in terms of their own (aspectual and temporal) meaning.
=> realis, potential (with ἄν), counterfactual
The usual translation is a subordinate clause, introduced by 'that'. However, it is sometimes preferable to use an indirect question when translating: ὁρῶμεν μὴ Νῑκίᾱς οἴεταί τι λέγειν 'Let us see if Nicias thinks he is talking sense.'
Sometimes ὅπως μή is used instead of μή.
[based on perseus under philologic: very rare]