Lloyd (The King Who Lost America, 1971; The Maligned Monarch, 1972) is a highly readable historical popularizer, several notches above the average practitioner of the genre. But his latest effort -- the story of the epic showdown between the armies of Athens, and Persia in 490 B.C. -- is a disappointment. To be sure the tale of Phidippides, the Athenian runner who delivered the tidings ""Athens is saved!"" before dropping dead on the ground after the 22-mile run from Marathon, is as exciting as it was when Herodotus first recounted it. But Lloyd's portentous subtitle, suggesting an inexorable clash between civilizations and cultures, is not borne out by his own narrative which makes it plain that the Athenians were to the Persians what the flea was to the elephant -- a damnable nuisance rather than an estimable foe. And instead of concentrating his story on the events and forces which directly preceded Marathon, Lloyd devotes half his book to cursory accounts of the rise and fall of Persia's imperial predecessors from Ur to Babylon. His portrait of the freedom-loving Greeks is better though occasionally he tends to idealize them -- as when he dubs the females of Sparta ""some of the best adjusted women in history."" Perhaps the moral is that Lloyd ought to stick to more modern times and personalities like George III or even King John who are more accessible than the myth-shrouded ancients.