The Greco-Roman world had another "world" or empire to its east, the Persians, with which there was constant interaction: Xenophon, The Anabasis, the March Up Country, the Greco-Persian wars, the famous battles of Marathon and Salamis, the Greek tragedy "The Persians" by Aeschylus, Alexander the Great's defeat of the Persian emperor Darius III and conquest of the Persian empire, or the later Roman generals' difficulties with the Persian armies, such as Pompey the Great, and of Marcus Licinius Crassus (conqueror of the slave general Spartacus), who was defeated in the field by a Persian force and was beheaded by them.