In regard to Thucydides' treatment of the period between the Persian and Peloponnesian Wars (the so-called _Pentecontaëteris_) it should be remembered that he does not profess to give, even in outline, the history of this period as a whole. The period is regarded simply as a prelude to the Peloponnesian War. There is no attempt to sketch the history of the Greek world or of Greece proper during this period. There is, indeed, no attempt to give a complete sketch of Athenian history. His object is to trace the growth of the Athenian Empire, and the causes that made the war inevitable. Much is therefore omitted not only in the history of the other Greek states, especially the Peloponnesian, but even in the history of Athens. Nor does Thucydides attempt an exact chronology. He gives us a few dates (e.g. surrender of Ithome, in the tenth year, i. 103; of Thasos, in the third year, i. 101; duration of the Egyptian expedition six years, i. 110; interval between Tanagra and Oenophyta 61 days, i. 108; revolt of Samos, in the sixth year after the Thirty Years' Truce, i. 115), but from these data alone it would be impossible to reconstruct the chronology of the period. In spite of all that can be gleaned from our other authorities, our knowledge of this, the true period of Athenian greatness, must remain slight and imperfect as compared with our knowledge of the next thirty years.
Of the secondary authorities for this period the two principal ones are Diodorus (xi. 38 to xii. 37) and Plutarch. Diodorus is of value chiefly in relation to Sicilian affairs, to which he devotes about a third of this section of his work and for which he is almost our sole authority. His source for Sicilian history is the Sicilian writer Timaeus (q.v.), an author of the 3rd century B.C. For the history of Greece Proper during the Pentecontaetia Diodorus contributes comparatively little of importance. Isolated notices of particular events (e.g. the _Synoecism_ of Elis, 471 B.C., or the foundation of Amphipolis, 437 B.C.), which appear to be derived from a chronological writer, may generally be trusted. The greater part of his narrative is, however, derived from Ephorus, who appears to have had before him little authentic information for this period of Greek history other than that afforded by Thucydides' work. Four of Plutarch's _Lives_ are concerned with this period, viz. _Themistocles_, _Aristides_, _Cimon_ and _Pericles_. From the _Aristides_ little can be gained. Plutarch, in this biography, appears to be mainly dependent upon Idomeneus of Lampsacus, an excessively untrustworthy writer of the 3rd century B.C., who is probably to be credited with the invention of the oligarchical conspiracy at the time of the battle of Plataea (ch. 13), and of the decree of Aristides, rendering all four classes of citizens eligible for the archonship (ch. 22). The _Cimon_, on the other hand, contains much that is valuable; such as, e.g. the account of the battle of the Eurymedon (chs. 12 and 13). To the _Pericles_ we owe several quotations from the Old Comedy. Two other of the _Lives_, _Lycurgus_ and _Solon_, are amongst our most important sources for the early history of Sparta and Athens respectively. Of the two (besides _Pericles_) which relate to the Peloponnesian War, _Alcibiades_ adds little to what can be gained from Thucydides and Xenophon; the _Nicias_, on the other hand, supplements Thucydides' narrative of the Sicilian expedition with many valuable details, which, it may safely be assumed, are derived from the contemporary historian, Philistus of Syracuse. Amongst the most valuable material afforded by Plutarch are the quotations, which occur in almost all the _Lives_, from the collection of Athenian decrees ([Greek: psêphismatôn sunagôgê]) formed by the Macedonian writer Craterus, in the 3rd century B.C. Two other works may be mentioned in connexion with the history of Athens. For the history of the Athenian Constitution down to the end of the 5th century B.C. Aristotle's _Constitution of Athens_ (q.v.) is our chief authority. The other _Constitution of Athens_, erroneously attributed to Xenophon, a tract of singular interest both on literary and historical grounds, throws a good deal of light on the internal condition of Athens, and on the system of government, both of the state and of the empire, in the age of the Peloponnesian War, during the earlier years of which it was composed.