At the beginning of the next year a calamity which no statesman could have foreseen overtook Athens. A mysterious plague of the greatest malignity scourged the city, the mortality being multiplied among the crowds of refugees. The city's strength was seriously impaired, public and private morality were undermined, inasmuch as none knew how long he had to live. Discouraged by it and by the invasions the Athenians sent a fruitless embassy to Sparta and tinned in fury on Pericles. He made a splendid defence of his policy and gave them heart to continue the struggle; he pointed out that it was better to lose their property and save the State than save their property and lose the State; their fleet opened to them the world of waters over which they could range as absolute masters. Soon afterwards he died, surviving the opening of the war only two years and a half; his character and abilities received due acknowledgment from Thucydides. At this point Sparta decided to destroy Plataea, the Athenian outpost in Boeotia. A very brilliant description of the siege and counter-operations reveals very clearly the Spartan inability to attack walled towns and explains their objection to fortified friends. Leaving the town guarded they retired for a time, to complete the work later. The war began to spread beyond the Peloponnese to the north of the Corinthian gulf, the control of which was important to both sides. The Acarnanians were attacked by Sparta and appealed to the Athenian admiral Phormio. Two naval actions in the gulf revealed the astonishing superiority of the Athenian navy on the high seas. Threatened in her corn supply in the west, Sparta began to intrigue with the outlying kingdoms on the north-east, the "Thraceward parts" on the trade-route being the objective. A spirit of revolt against Athenian rule appeared in Lesbos, which seceded in 428. The chief town in this non-Ionic island was Mytilene, which sent ambassadors to Sparta. Their speech clearly explains how the Athenians were able to keep their hold on their policy; her policy (like that of Rome) was to divide the allies by carefully grading their privileges, playing off the weak against the stronger. The Spartans proved unable to help and the Athenians easily blockaded the city, capturing it early in 427. In their anger they at first decided to slay all the inhabitants, but a better feeling led to a reconsideration next day. In the Assembly two great speeches were delivered. Pericles had been succeeded by Cleon, to whom Thucydides seems to have been a little unjust. He opened his speech with the famous remark that a democracy cannot govern an Empire; it is liable to sudden fits of passion which make a consistent policy impossible. He himself never changed his plans, but his audience were different.