The ancient historian who wrote the Anabasis of Alexander was Arrian. His full name was Lucius Flavius Arrianus, a Greek-speaking author from Nicomedia who lived in the second century AD, roughly four centuries after Alexander's conquests.
Arrian's seven-book history follows Alexander's campaigns from his accession in 336 BC through his death in 323 BC. It is chiefly a military narrative, covering battles, sieges, diplomacy, geography, and the organization of Alexander's empire.
Arrian drew heavily on now-lost accounts by Alexander's generals Ptolemy and Aristobulus, as well as the admiral Nearchus. He deliberately modelled the work's title and structure on Xenophon's Anabasis, which described an earlier Greek military expedition.
A common mix-up is with Plutarch, who wrote a famous Life of Alexander. Plutarch's biography focuses more on character and moral anecdotes, while Arrian's Anabasis is the fullest surviving narrative of Alexander's campaigns. Scholars value it highly but also note that Arrian sometimes presents Alexander too favorably.