Greek history began with the epic and genealogical writers like Homer and Hesiod; Herodotus followed with his prose epic of the Persian Wars. Greek history became scientific and critical with Thucydides but did not remain so after him: from Xenophon onwards it lapsed into rhetoric and romance. Roman historians followed Greek principles and methods, and the earliest wrote in Greek. Polybius alone cared much for scientific criticism in his work; the rest held, with Cicero, that history is a branch of the art of rhetoric. Livy and Tacitus produced works of great literary merit at the expense of historical accuracy. Julius Caesar's account of his conquest of Gaul was the military report of a general written to the people who had given him his commission, to justify his unconstitutional action in conquering Gaul without instructions. These are only the foremost names, but of all Greek and Roman historians only Thucydides and Polybius used in writing the scientific and critical standards of modern historians. The others wrote to please the public taste for rhetoric, to grind some axe, to edify their readers, to relate anecdotes, to praise or blame, to express personal opinions, to display their literary powers, to make digressions, and not many were saved by literary skill from the mediocrity to which their critical weakness doomed them.