Megabazos’ campaign in Thrace was such a step. But when this is said, it must not be supposed that the remark is intended to convey the impression that this was the sole or even the main motive of that campaign. Sole motives are, as it has been said, rare in the history of policies. The design on Greece was the corollary to the problem to be solved in Thrace. It was not necessarily regarded as either superior or inferior in importance to the Thracian problem. It is possible that the designs on both Thrace and Greece were parts of a larger design which aimed at a simultaneous advance of the Persian frontier along both sides of the Mediterranean. It is, at any rate, noticeable that at the time of these operations in Europe, if Herodotus’ rough chronology is to be accepted, Aryandes, the Persian satrap in Egypt, was engaged in a campaign in Cyrene. ◆H. iii. 13; iv. 165.◆ Cyrene had been tributary to Persia for some time past; but troubles had arisen there which called for Persian interference; and this resulted in the Persian frontier being advanced as far as Euesperides. There still remains what is, owing to the lack of evidence, the very difficult question as to the reasons which prevented or dissuaded Darius from carrying out his designs on Greece in the years which intervened between the campaign of Megabazos and the outbreak of the Ionian revolt. That may be calculated to have actually broken out in 499. But what is the date of the end of Megabazos’ campaign? ◆H. v. 23.◆ It certainly took place some time before Darius left Sardes. Megabazos finds him there on his return; and Histiæus is subsequently recalled to Sardes before Darius leaves the place. Unfortunately, it is not possible to fix the date of Darius’ departure from Sardes. ◆H. v. 73.◆ He had certainly gone to Susa before the arrival of the Athenian embassy which appealed to Persia for aid against Spartan aggression, because it is to Artaphernes that the appeal is addressed. But even if the date of Hippias’ expulsion be taken as fixed, who is to say what time elapsed between that and the sending of this embassy? ◆H. v. 66, 70, 72, 73.◆ Much happened at Athens during the interval—the political struggle between Kleisthenes and Isagoras; the demand of Kleomenes of Sparta for the expulsion of Kleisthenes; the retirement of Kleisthenes; the disastrous expedition of Kleomenes, ending in his own defeat, and in disaster to the party of Isagoras; the return of Kleisthenes, and the carrying out of the reforms which bear his name.