Anabasis is the most famous work, in seven books, of the Greek professional soldier and writer Xenophon. The journey it narrates is his best known accomplishment and "one of the great adventures in human history," as Will Durant expressed the common assessment. However, although the content of the book is lively, written in the style of someone who has participated in the adventures he describes, the story recounted in the Anabasis is completely uncorroborated.

Xenophon accompanied the Ten Thousand, a large army of Greek mercenaries hired by Cyrus the Younger, who intended to seize the throne of Persia from his brother, Artaxerxes II. Though Cyrus' mixed army fought to a tactical victory at Cunaxa in Babylon (401 BC), Cyrus was killed, rendering the actions of the Greeks irrelevant and the expedition a failure.

Stranded deep in Persia, the Spartan general Clearchus and the other Greek senior officers were then killed or captured by treachery on the part of the Persian satrap Tissaphernes. Xenophon, one of three remaining leaders elected by the soldiers, played an instrumental role in encouraging the 10,000 to march north across foodless deserts and snow-filled mountain passes, towards the Black Sea and the comparative security of its Greek shoreline cities. Now abandoned in northern Mesopotamia, without supplies other than what they could obtain by force or diplomacy, the 10,000 had to fight their way northwards through Corduene and Armenia, making ad hoc decisions about their leadership, tactics, provender and destiny, while the King's army and hostile natives barred their way and attacked their flanks.

Ultimately this "marching republic" managed to reach the shores of the Black Sea at Trabzon (Trebizond), a destination they greeted with their famous cry of exultation on the mountain of Theches (now Madur) in Sürmene : "thálatta, thálatta", "the sea, the sea!". "The sea" meant that they were at last among Greek cities but it was not the end of their journey, which included a period fighting for Seuthes II of Thrace and ended with their recruitment into the army of the Spartan general Thibron. Xenophon related this story in Anabasis in a simple and direct manner.

The Greek term anabasis referred to an expedition from a coastline into the interior of a country. The term katabasis referred to a trip from the interior to the coast. While the journey of Cyrus is an anabasis from Ionia on the eastern coast of the Aegean Sea, to the interior of Asia Minor and Mesopotamia, most of Xenophon's narrative is taken up with the return march of Xenophon and the Ten Thousand, from the interior of Babylon to the coast of the Black Sea. Socrates makes a cameo appearance, when Xenophon asks whether he ought to accompany the expedition. The short episode demonstrates the reverence of Socrates for the Oracle of Delphi.

Xenophon's account of the exploit resounded through Greece, where, two generations later, some surmise, it may have inspired Philip of Macedon to believe that a lean and disciplined Hellene army might be relied upon to defeat a Persian army many times its size.
Besides military history, the Anabasis has found use as a tool for the teaching of classical philosophy; the principles of leadership and government exhibited by the army can be seen as exemplifying Socratic philosophy.