From, A History of Greek Philosophy, vol. IV, Plato: the man and his dialogues, earlier period,
Cambridge University Press, 19896, pp. 8-38.
Ιt was not long before the Thirty fell, and under the succeeding democracy Plato again felt, though with more hesitation, the stirrings of desire to take part in public and political affairs. Inevitably there were acts of vengeance on political opponents, and others which he could not approve, but he acknowledges that on the whole the returned exiles showed considerable moderation. Unfortunately one of their mistakes was to execute Socrates, who was not only Plato’s friend and in his opinion ‘the most righteous man then living’, but a man who had defied the wrath of the Thirty to befriend one οf their own number when they themselves were in the wilderness. As he brooded not only on this devastating loss but also on the kind of men who held political control, and on the laws and customs in general, on the necessity of personal connexions for success and the growth of corrupt practices, the young man who had started out all eagerness for a political career felt dizzy and confused. He did not give υp all hope of an improvement, but ‘was always waiting for the right moment to act’. It is not surprising, as Cornford pointed out, that for such a man the right moment never came. ‘The whole of this long letter reveals - what we might guess from his other writings - that his power and gifts were of such a kind that he could never be a leading man of action in the society of his time.’ His only conclusion was, he says, the scarcely practical one to which he gave expression in the Republic, that the troubles of the human race will never cease until either philosophers in possession of rightness and truth attain political power or those who have the power become ‘by some dispensation of divine providence’ genuine philosophers.
(d) Sicily and the Academy 
HIS was my frame of mind’, the letter continues (326b), ‘when Ι first came to Italy and Sicily.’ Earlier Plato has mentioned that he was forty at the time, i.e. it was about the year 387 B.C.. He gives no reason for going, but his motive in the case of Italy was probably what later writers said, namely a desire to make personal contact with the Pythagorean philosophers settled there, and notably with Archytas the philosopher-statesman of Tarentum, to whose friendly relations with Plato the Seventh Letter itself bears witness. The political instability of the Italian Greeks, and their conception of la dolce vita, were a shock to him, and provided further food for thought as he crossed to Sicily. In doing this, he himself seems to have had no definite purpose in mind, and later authorities were reduced to alleging a desire to see the craters and lava-flow of Etna. The letter itself simply says that it was ‘perhaps chance, though it looks as if some higher power contrived it to start the train of events concerning Dion and Syracuse’ (326e). Once there, one momentous event in the visit so eclipsed all others in his mind that he mentions nothing else, not even the name of the tyrant Dionysius Ι. This was his meeting with Dion, then aged about twenty, to whom he became passionately attached, a meeting whose fateful consequences were all in his mind when the Seventh Letter was written. Dion’s connexions with the tyrant were close. His sister Aristomache was married to Dionysius, and he himself married his own niece, Aristomache’s daughter. Plato describes him as a youth of exceptional intellectual and moral gifts, the perfect pupil to whom he could open his heart about his own political ideals. Dion eagerly absorbed his Socratic teaching of the superiority of virtue to pleasure and luxury, and renounced the lax habits of the Italiotes and Siceliotes - thereby bringing on himself a certain unpopularity in court circles so long as Dionysius Ι was alive.
A Day in Old Athens * A Short History of Greek Philosophy
The Greek Word Library
Reference address : http://www.ellopos.net/elpenor/greek-texts/ancient-greece/guthrie-plato.asp?pg=9