Subjunctive conditions in [21β, p. 83] explained 22a, p. 93-5. 
 
ἐὰν δὲ εἰς πόλεμον καταστῶμεν,...
νικήσομεν [=τὰ τοῦ πολέμου οὐκ ἀσθενέστερα ἕξομεν]
 
ἡμεῖς γὰρ τῆς θαλάσσης κρατοῦμεν. 
καὶ ἐὰν ἐπὶ τὴν χώραν ἡμῶν πεζῇ ἴωσιν 
                                                   [= εἰσβάλλωσιν invade]
          ...ἡμεῖς ἐπὶ τὴν [χώραν] ἐκείνων πλευσόμεθα..
μέγα γάρ ἐστι τὸ τῆς θαλάσσης κράτος.
 
νῦν δὲ τούτοις ἀποκρινάμενοι ἀποπέμψωμεν, ὅτι
           τὰς πόλεις αὐτονόμους ἀφήσομεν,
                    ἐὰν καὶ εκεῖνοι ἀφῶσι τὰς πόλεις 
                                                             ἂς ὑπηκόους ἔχουσιν...
 
In addition to the general conditions (p.94), much the same more-vivid idea
can be expressed with ὅστις-οἵτινες ἄν   = 'whoever/ if anyone'
 
(from above) 
οἵτινες ἂν ἐπὶ τὴν χώραν ἡμῶν πεζῇ ἴωσιν (whoever invades our land)
                                             ...ἡμεῖς ἐπὶ τὴν [χώραν] ἐκείνων πλευσόμεθα..
 
1) Whoever listens will learn
                    ὅστις ἂν ἀκούῃ, μαθήσεται, κτλ.
 
2) Whoever gives will receive.
 
 
3)  Whoever comes will go.