The fundamental question is this: Was Mytiline "justified" rebelling against Athens? Kagan believes so:

The request of Mytilene [for unification] and the rebellion that followed its refusal were completely unjustified and the Athenian response necessary by any reasonable judgment.

Diodorus: On Mytilene: 12:55

Κατὰ δὲ τὴν Ἑλλάδα Λέσβιοι μὲν ἀπέστησαν ἀπὸ τῶν Ἀθηναίων· ἐνεκάλουν γὰρ αὐτοῖς, ὅτι βουλομένων συνοικίζειν πάσας τὰς κατὰ τὴν Λέσβον πόλεις εἰς τὴν Μυτιληναίων πόλιν διεκώλυσαν. διὸ καὶ πρὸς Λακεδαιμονίους ἀποστείλαντες πρεσβευτὰς καὶ συμμαχίαν συνθέμενοι συνεβούλευον τοῖς Σπαρτιάταις ἀντέχεσθαι τῆς κατὰ θάλατταν ἡγεμονίας· πρὸς ταύτην δὲ τὴν ἐπιβολὴν ἐπηγγείλαντο πολλὰς τριήρεις εἰς τὸν πόλεμον παρέξεσθαι. 3ἀσμένως δὲ τῶν Λακεδαιμονίων ὑπακουσάντων καὶ περὶ τὴν κατασκευὴν τῶν τριήρων γινομένων, Ἀθηναῖοι φθάσαντες αὐτῶν τὴν παρασκευὴν παραχρῆμα δύναμιν ἐξέπεμψαν εἰς τὴν Λέσβον, πληρώσαντες ναῦς τετταράκοντα καὶ στρατηγὸν προχειρισάμενοι Κλεινιππίδην. οὗτος δὲ προσλαβόμενος βοήθειαν παρὰ τῶν συμμάχων κατέπλευσεν εἰς Μυτιλήνην. γενομένης δὲ ναυμαχίας οἱ μὲν Μυτιληναῖοι λειφθέντες συνεκλείσθησαν εἰς πολιορκίαν, τῶν δὲ Λακεδαιμονίων ψηφισαμένων βοηθεῖν τοῖς Μυτιληναίοις καὶ παρασκευαζομένων στόλον ἀξιόλογον, ἔφθασαν Ἀθηναῖοι ναῦς ἄλλας σὺν ὁπλίταις χιλίοις ἀποστείλαντες εἰς Λέσβον. τούτων δ᾿ ἡγούμενος Πάχης ὁ Ἐπικλήρου καταντήσας εἰς τὴν Μυτιλήνην, καὶ τὴν προϋπάρχουσαν δύναμιν παραλαβών, περιετείχισε τὴν πόλιν καὶ συνεχεῖς προσβολὰς ἐποιεῖτο οὐ μόνον κατὰ γῆν, ἀλλὰ καὶ κατὰ θάλατταν. Λακεδαιμόνιοι δὲ ἐξαπέστειλαν εἰς τὴν Μυτιλήνην τριήρεις μὲν τετταράκοντα πέντε καὶ στρατηγὸν Ἀλκίδαν, εἰς δὲ τὴν Ἀττικὴν εἰσέβαλον μετὰ τῶν συμμάχων· ἐπελθόντες δὲ τοὺς παραλελειμμένους τόπους τῆς Ἀττικῆς καὶ δῃώσαντες τὴν χώραν ἐπανῆλθον εἰς τὴν οἰκείαν. Μυτιληναῖοι δὲ τῇ σιτοδείᾳ καὶ τῷ πολέμῳ πιεζόμενοι καὶ στασιάζοντες πρὸς ἀλλήλους, καθ᾿ ὁμολογίαν παρέδωκαν 8τὴν πόλιν τοῖς πολιορκοῦσιν. ἐν δὲ ταῖς Ἀθήναις. τοῦ δήμου βουλευομένου πῶς χρὴ προσενέγκασθαι τοῖς Μυτιληναίοις, Κλέων ὁ δημαγωγός, ὠμὸς ὢν τὸν τρόπον καὶ βίαιος, παρώξυνε τὸν δῆμον, ἀποφαινόμενος δεῖν τοὺς Μυτιληναίους αὐτοὺς μὲν ἡβηδὸν ἅπαντας ἀποκτεῖναι, τέκνα δὲ καὶ γυναῖκας ἐξανδραποδίσασθαι. τέλος δὲ πεισθέντων τῶν Ἀθηναίων κατὰ τὴν γνώμην τε τοῦ Κλέωνος ψηφισαμένων, ἀπεστάλησαν εἰς τὴν Μυτιλήνην οἱ τὰ δοχθέντα τῷ δήμῳ δηλώσοντες τῷ στρατηγῷ. τοῦ δὲ Πάχητος ἀναγνόντος τὸ ψήφισμα ἦλθεν ἐναντίον τῷ προτέρῳ ἕτερον. ὁ δὲ Πάχης γνοὺς τὴν μετάνοιαν τῶν Ἀθηναίων ἐχάρη, καὶ τοὺς Μυτιληναίους συναγαγὼν εἰς ἐκκλησίαν ἀπέλυσε τῶν ἐγκλημάτων, ἅμα δὲ καὶ τῶν μεγίστων φόβων. Ἀθηναῖοι δὲ τῆς Μυτιλήνης τὰ τείχη περιελόντες τὴν Λέσβον ὅλην πλὴν τῆς Μηθυμναίων χώρας κατεκληρούχησαν. Ἡ μὲν οὖν Λεσβίων ἀπόστασις ἀπ᾿ Ἀθηναίων τοιοῦτον ἔσχε τὸ τέλος.

In Greece the Lesbians revolted from the Athenians; for they harboured against them the complaint that, when they wished to merge all the cities of Lesbos with the city of the Mytilenaeans, the Athenians had prevented it. Consequently, after dispatching ambassadors to the Peloponnesians and concluding an alliance with them, they advised the Spartans to make an attempt to seize the supremacy at sea, and toward this design they promised to supply many triremes for the war. The Lacedaemonians were glad to accept this offer, but while they were busied with the building of the triremes, the Athenians forestalled their completion by sending forthwith a force against Lesbos, having manned forty ships and chosen Cleinippides as their commander, He gathered reinforcements from the allies and put in at Mytilenê. In a naval battle which followed the Mytilenaeans were defeated and enclosed within a siege of their city. Meanwhile the Lacedaemonians had voted to send aid to the Mytilenaeans and were making ready a strong fleet, but the Athenians forestalled them by sending to Lesbos additional ships along with a thousand hoplites. Their commander, Paches the son of Epiclerus, upon arriving at Mytilenê, took over the force already there, threw a wall about the city, and kept launching continuous assaults upon it not only by land but by sea as well. The Lacedaemonians sent forty-five triremes to Mytilenê under the command of Alcidas, and they also invaded Attica together with their allies; here they visited the districts of Attica which they had passed by before, ravaged the countryside, and then returned home. And the Mytilenaeans, who were distressed by lack of food and the war and were also quarrelling among themselves, formally surrendered the city to the besiegers. While in Athens the people were deliberating on what action they should take against the Mytilenaeans, Cleon, the leader of the populace and a man of cruel and violent nature, spurred on the people, declaring that they should slay all the male Mytilenaeans from the youth upward and sell into slavery the children and women. In the end the Athenians were won over and voted as Cleon had proposed, and messengers were dispatched to Mytilene to make known to the general the measures decreed by the popular assembly. Even as Paches had finished reading the decree a second decree arrived, the opposite of the first. Paches was glad when he learned that the Athenians had changed their minds, and gathering the Mytilenaeans in assembly he declared them free of the charges as well as of the greatest fears. The Athenians pulled down the walls of Mytilene and portioned out in allotments the entire island of Lesbos with the exception of the territory of the Methymnaeans. Such, then, was the end of the revolt of the Lesbians from the Athenians.