Quintus Curtius Rufus, History of Alexander the Great, 7.3.28-35, on the way to the River Tanais:
perventum erat in parvulum oppidum. Branchidae eius incolae erant.
Mileto quondam iussu Xerxis, cum e Graecia rediret, transierant, et in
ea sede constiterant, quia templum, quod Didymeon appellatur, in gratiam
Xerxis violaverant.  mores patrii nondum exoleverant; sed iam bilingues
erant paulatim a domestico externo sermone degeneres. magno igitur
gaudio regem excipiunt urbem seque dedentes. Ille Milesios, qui apud
ipsum militarent, convocari iubet.  vetus odium Milesii gerebant in
Branchidarum gentem. proditis ergo, sive iniuriae, sive originis
meminisse mallent, liberum de Branchidis permittit arbitrium.
 variantibus deinde sententiis se ipsum consideraturum quid optimum factu
esset ostendit. Postero die occurrentibus Branchidis secum procedere
iubet; cumque ad urbem ventum esset, ipse cum expedita manu portam
intrat.  phalanx moenia oppidi circumire iussa, et dato signo diripere
urbem proditorum receptaculum, ipsosque ad unum caedere.  illi inermes
passim trucidantur, nec aut commercio linguae, aut supplicum velamentis
precibusque inhiberi crudelitas potest. tandem ut deicerent fundamenta
murorum ab imo moliuntur, ne quod urbis vestigium exstaret.  nemora
quoque et lucos sacros non caedunt modo, sed etiam exstirpant, ut vasta
solitudo et sterilis humus excussis etiam radicibus linqueretur.  quae si
in ipsos proditionis auctores excogitata essent, iusta ultio esse, non
crudelitas videretur; nunc culpam maiorum posteri luere, qui ne viderant
quidem Miletum, ideo et Xerxi non potuerant prodere.
"In pursuit of Bessus they had arrived at a small town inhabited by the Branchidae who, on the orders of Xerxes, when he was returning from Greece, had emigrated from Miletus and settled in this spot. This was necessary because, to please Xerxes, they had violated the temple called the Didymeon.  The culture of their forebears had not yet disappeared, though they were now bilingual and the foreign tongue was gradually eroding their own. So it was with great joy that they welcomed Alexander to they surrendered themselves and their city. Alexander called a meeting of the Milesians in his force,  for the Milesians bore a long-standing grudge against the Branchidae as a clan. Since they were the people betrayed by the Branchidae, Alexander let them decide freely on their case, asking if they preferred to remember their injury or their common origin.  But when there was a difference of opinion over this, he declared that he would himself consider the best course of action. When the Branchidae met him the next day, he told them to accompany him. On reaching the city, he himself entered through the gate with a unit of light-armed troops.  The phalanx had been ordered to surround the city walls and, when the signal was given, to sack the city which provided refuge for traitors, killing the inhabitants to a man.  The Branchidae, who were unarmed, were butchered throughout the city, and neither community of language nor the olive-branches and entreaties of the suppliants could curb the savagery. Finally the Macedonians dug down to the foundations of the walls in order to demolish them and leave not a single trace of the city.  Woods, too, and sacred groves, they not only cut down but actually uprooted, so that nothing would remain after the removal of the roots but empty wasteland and barren soil.  Had this punishment been devised against the people responsible for the treachery, it might have appeared to be fair revenge rather than brutality but, as it was, the guilt of their ancestors was being atoned for by descendants who had not even seen Miletus and accordingly could not possibly have betrayed it to Xerxes."
(translation: Yardley, Penguin)