The tyrant, with his characteristic energy, prepared to meet his enemy in Macedonia, but the king anticipated him, and, reaching Larissa, was admitted into the city, obliged the Thessalian Alexander to flee to Pherae, and left a garrison in Larissa, as well as in Crannon, which had also come over to him. But the Macedonian having retired, his friends in Thessaly feared the vengeance of Alexander, and sent for aid to Thebes, the policy of which was to check a neighbor who might otherwise become formidable, and Pelopidas was accordingly dispatched to aid them. On the arrival of the latter at Larissa, whence according to Diodorus he dislodged the Macedonian garrison, Alexander presented himself and offered submission; but soon after escaped by flight, alarmed by the indignation which Pelopidas expressed at the tales he heard of his cruelty and tyrannical profligacy.
These events appear to be referable to the early part of the year 368 BC. In the summer of that year Pelopidas was again sent into Thessaly, in consequence of fresh complaints against Alexander. Accompanied by Ismenias, he went merely as a negotiator, and without any military force, and venturing incautiously within the power of the tyrant, was seized by him and thrown into prison. The scholar William Mitford suggested that Pelopidas was taken prisoner in battle, but the language of Demosthenes hardly supports such an inference. The Thebans sent a large army into Thessaly to rescue Pelopidas, but they could not keep the field against the superior cavalry of Alexander, who, aided by auxiliaries from Athens, pursued them with great slaughter; and the destruction of the whole Theban army is said to have been averted only by the ability of Epaminondas, who was serving in the campaign, but not as general.
The next year, 367 BC, was initiated by a specimen of Alexander's treacherous cruelty, in the massacre of the citizens of Scotussa; and also by another expedition of the Thebans under Epaminondas into Thessaly, to rescue Pelopidas. According to Plutarch, Alexander did not dare offer resistance, and was glad to purchase even a thirty days' truce by the delivery of the prisoners. During the next three years Alexander would seem to have renewed his attempts against the states of Thessaly, especially those of Magnesia and Phthiotis, for at the end of that time, 364 BC, we find them again applying to Thebes for protection against him. The army appointed to march under Pelopidas is said to have been dismayed by an eclipse (on June 13, 364), and Pelopidas, leaving it behind, entered Thessaly at the head of three hundred volunteer horsemen and some mercenaries. A battle ensued at Cynoscephalae, wherein Pelopidas was himself slain, but defeated Alexander; and this victory was closely followed by another of the Thebans under Malcites and Diogiton, who obliged Alexander to restore the conquered towns to the Thessalians, to confine himself to Pherae, to join the Boeotian League, and to be a dependent ally of Thebes.
If the death of Epaminondas in 362 BC freed Athens from fear of Thebes, it appears at the same time to have exposed her to further aggression from Alexander, who made a piratical raid on Tinos and other cities of the Cyclades, plundering them, and making slaves of the inhabitants. He besieged Peparethus too, and "even landed troops in Attica itself, and seized the port of Panormus, a little eastward of Sounion." Leosthenes, the Athenian admiral, defeated him, and relieved Peparethus, but Alexander delivered his men from a blockade in Panormus, took several Attic triremes, and plundered the Piraeus.
Of Thebe's motive for the murder different accounts are given. Plutarch states it to have been fear of her husband, together with hatred of his cruel and brutal character, and ascribes these feelings principally to the representations of Pelopidas, when she visited him in his prison. In Cicero the deed is ascribed to jealousy. Other accounts have it that Alexander had taken Thebe's youngest brother as his eromenos and tied him up. Exasperated by his wife's pleas to release the youth, he murdered the boy, which drove her to revenge.