In November 333, Alexander defeated the Persian king Darius III Codomannus. After this battle of Issus, Darius offered to surrender half of his empire. The Macedonian king considered this not enough; the Greek author Arrian of Nicomedia describes Alexander's reply in chapter 2.14 of his Anabasis. The text of the reply is probably not authentic, but contains the general gist of Alexander's letter. The translation was made by M.M. Austin.
Alexander's Letter to Darius III
[2.14.1] While Alexander was still at Marathus there came to him envoys from Darius with a letter from him and a request they conveyed verbally to release Darius' mother, wife and children.
[2.14.2] The letter's contents were to the effect that there had been friendship and alliancenote[No alliance between Philip of Macedonia and Artaxerxes III is known from other sources.] between Philip and Artaxerxesnote[Artaxerses III.] but when Artaxerxes' son Arsesnote[Arses ruled under the name Artaxerxes IV (July 338 - Summer 336).] came to the throne, Philip was the initial aggressor against Arses, though he had suffered no harm from the Persians. Since Darius' own accession Alexander had not sent any envoy to confirm the old friendship and alliance, but crossed with an army to Asia and did the Persians much harm.
[2.14.3] This was why Darius had come downnote[From Persia to Issus.] to defend his country and preserve his ancestral rule. The battle had been decided as some god willed, but he, as one king to another, was asking for his captive wife, mother and children, and was prepared to make a treaty of friendship with Alexander and to be his ally. He requested Alexander to send envoys to accompany the Persian emissaries Meniscus and Arsimas and to exchange mutual assurances.
[2.14.4] Alexander drafted a reply to this letter and sent Thersippus to accompany the envoys from Darius, with instructions to hand over the letter to Darius but not to engage in any negotiations. Alexander's letter read as follows: "Your ancestors invaded Macedonianote[Macedonia had been made a Persian vassal in c. 512 by the Persian commander Megabazus. In 492, Mardonius had strengthened the Persian grip on Macedonia.] and the rest of Greecenote[In 490, Darius had send an army, commanded by Datis and Artaphernes, to conquer the Greek islands in the Aegean sea. It had seized them all and had deported the inhabitants of Eretria. Later, Datis had tried to make Hippias, a pro-Persian nobleman, tyrant of Athens, but his army had been defeated near Marathon. (Herodotus wrote about it; text.)] and did us harm although we had not done you any previous injury. I have been appointed commander-in-chief of the Greeks and it is with the aim of punishing the Persians that I have crossed into Asia, since you are the aggressors.
[2.14.5] You gave support to the people of Perinthus, who had done my father harm, and Ochusnote[Surname of Artaxerxes III.] sent a force to Thrace, which was under our rule. My father died at the hand of conspirators instigated by you,note[There is no proof for this accusation.] as you yourself boasted to everybody in your letters, you killed Arsesnote[Arses ruled under the name Artaxerxes IV (July 338 - Summer 336).] with the help of Bagoasnote[According to Diodorus of Sicily, the Persian courtier Bagoas had poisoned Artaxerxes III Ochus (338) and Artaxerxes IV Arses (336) (more). He had seen to the accession of Darius III, who had had the man executed. It is difficult to understand what really happened.] and gained your throne through unjust means, in defiance of Persian custom and doing wrong to the Persians. You sent unfriendly letters to the Greeks about me, to push them to war against me,
[2.14.6] and sent money to the Spartans and some other Greeks, which none of the other cities would accept apart from the Spartans. Your envoys corrupted my friends and sought to destroy the peace which I established among the Greeks.note[A reference to the Corinthian league in which the Greek cities were gathered.] I therefore led an expedition against you, and you started the quarrel.
[2.14.7] But now I have defeated in battle first your generals and satraps, and now you in person and your army, and by the grace of the gods I control the country. All those who fought on your side and did not die in battle but came over to me, I hold myself responsible for them; they are not on my side under duress but are taking part in the expedition of their own free will.
[2.14.8] Approach me therefore as the lord of all Asia. If you are afraid of suffering harm at my hands by coming in person, send some of your friends to receive proper assurances. Come to me to ask and receive your mother, your wife, your children and anything else you wish. Whatever you can persuade me to give shall be yours.
[2.14.9] In future whenever you communicate with me, send to me as king of Asia; do not write to me as an equal, but state your demands to the master of all your possessions. If not, I shall deal with you as a wrongdoer. If you wish to lay claim to the title of king, then stand your ground and fight for it; do not take to flight, as I shall pursue you wherever you may be."